diff --git a/src/Services/ImageConverter.php b/src/Services/ImageConverter.php
index b7659434..af947057 100644
--- a/src/Services/ImageConverter.php
+++ b/src/Services/ImageConverter.php
@@ -18,7 +18,6 @@ class ImageConverter
 		$this->imageManipulator = $imageManipulator;
 	}
 
-
 	public function createImageFromBuffer($source)
 	{
 		$tmpSourcePath = tempnam(sys_get_temp_dir(), 'thumb') . '.dat';
diff --git a/src/Services/ImageManipulation/GdImageManipulator.php b/src/Services/ImageManipulation/GdImageManipulator.php
index 276355b1..9f8aa58d 100644
--- a/src/Services/ImageManipulation/GdImageManipulator.php
+++ b/src/Services/ImageManipulation/GdImageManipulator.php
@@ -81,6 +81,7 @@ class GdImageManipulator implements IImageManipulator
 				break;
 
 			default:
+				throw new \InvalidArgumentException('Not supported');
 		}
 
 		$buffer = ob_get_contents();
diff --git a/src/Services/ThumbnailGenerator.php b/src/Services/ThumbnailGenerator.php
index 4e576d4f..f948a11e 100644
--- a/src/Services/ThumbnailGenerator.php
+++ b/src/Services/ThumbnailGenerator.php
@@ -1,6 +1,5 @@
 <?php
 namespace Szurubooru\Services;
-use Szurubooru\Services\ImageManipulation\IImageManipulator;
 use Szurubooru\Services\ImageManipulation\ImageManipulator;
 
 class ThumbnailGenerator
@@ -17,7 +16,7 @@ class ThumbnailGenerator
 		$this->imageConverter = $imageConverter;
 	}
 
-	public function generate($source, $width, $height, $cropStyle)
+	public function generate($source, $width, $height, $cropStyle, $format)
 	{
 		$image = $this->imageConverter->createImageFromBuffer($source);
 		if (!$image)
@@ -40,9 +39,7 @@ class ThumbnailGenerator
 				throw new \InvalidArgumentException('Unknown thumbnail crop style');
 		}
 
-		return $this->imageManipulator->saveToBuffer(
-			$image,
-			IImageManipulator::FORMAT_JPEG);
+		return $this->imageManipulator->saveToBuffer($image, $format);
 	}
 
 	private function cropOutside(&$image, $srcWidth, $srcHeight, $dstWidth, $dstHeight)
diff --git a/src/Services/ThumbnailService.php b/src/Services/ThumbnailService.php
index d9b3d7ba..f0508576 100644
--- a/src/Services/ThumbnailService.php
+++ b/src/Services/ThumbnailService.php
@@ -2,10 +2,15 @@
 namespace Szurubooru\Services;
 use Szurubooru\Config;
 use Szurubooru\Dao\PublicFileDao;
+use Szurubooru\Helpers\ProgramExecutor;
+use Szurubooru\Services\ImageManipulation\IImageManipulator;
 use Szurubooru\Services\ThumbnailGenerator;
 
 class ThumbnailService
 {
+	const PROGRAM_NAME_JPEGOPTIM = 'jpegoptim';
+	const PROGRAM_NAME_OPTIPNG = 'optipng';
+
 	private $config;
 	private $fileDao;
 	private $thumbnailGenerator;
@@ -59,11 +64,13 @@ class ThumbnailService
 		$source = $this->fileDao->load($sourceName);
 		if ($source)
 		{
-			$thumbnailContent = $this->thumbnailGenerator->generate($source, $width, $height, $cropStyle);
+			$format = $this->getFormat();
+			$thumbnailContent = $this->thumbnailGenerator->generate($source, $width, $height, $cropStyle, $format);
 			if ($thumbnailContent)
 			{
 				$targetName = $this->getTargetName($sourceName, $width, $height);
 				$this->fileDao->save($targetName, $thumbnailContent);
+				$this->optimize($targetName, $format);
 				return $targetName;
 			}
 		}
@@ -97,4 +104,35 @@ class ThumbnailService
 	{
 		return 'thumbnails' . DIRECTORY_SEPARATOR . 'blank.png';
 	}
+
+	private function getFormat()
+	{
+		return IImageManipulator::FORMAT_JPEG;
+	}
+
+	private function optimize($sourceName, $format)
+	{
+		$sourcePath = $this->fileDao->getFullPath($sourceName);
+
+		if ($format === IImageManipulator::FORMAT_JPEG and ProgramExecutor::isProgramAvailable(self::PROGRAM_NAME_JPEGOPTIM))
+		{
+			ProgramExecutor::run(
+				self::PROGRAM_NAME_JPEGOPTIM,
+				[
+					'--quiet',
+					'--strip-all',
+					$sourcePath,
+				]);
+		}
+
+		elseif ($format === IImageManipulator::FORMAT_PNG and ProgramExecutor::isProgramAvailable(self::PROGRAM_NAME_OPTIPNG))
+		{
+			ProgramExecutor::run(
+				self::PROGRAM_NAME_OPTIPNG,
+				[
+					'-o2',
+					$sourcePath,
+				]);
+		}
+	}
 }
diff --git a/tests/Services/ThumbnailGeneratorTest.php b/tests/Services/ThumbnailGeneratorTest.php
index 3dc4abb7..1ec32cf5 100644
--- a/tests/Services/ThumbnailGeneratorTest.php
+++ b/tests/Services/ThumbnailGeneratorTest.php
@@ -3,6 +3,7 @@ namespace Szurubooru\Tests\Services;
 use Szurubooru\Helpers\ProgramExecutor;
 use Szurubooru\Injector;
 use Szurubooru\Services\ImageConverter;
+use Szurubooru\Services\ImageManipulation\IImageManipulator;
 use Szurubooru\Services\ImageManipulation\ImageManipulator;
 use Szurubooru\Services\ThumbnailGenerator;
 use Szurubooru\Tests\AbstractTestCase;
@@ -24,7 +25,8 @@ final class ThumbnailGeneratorTest extends AbstractTestCase
 			$this->getTestFile('flash.swf'),
 			150,
 			150,
-			ThumbnailGenerator::CROP_OUTSIDE);
+			ThumbnailGenerator::CROP_OUTSIDE,
+			IImageManipulator::FORMAT_PNG);
 
 		$image = $imageManipulator->loadFromBuffer($result);
 		$this->assertEquals(150, $imageManipulator->getImageWidth($image));
@@ -46,7 +48,8 @@ final class ThumbnailGeneratorTest extends AbstractTestCase
 			$this->getTestFile('video.mp4'),
 			150,
 			150,
-			ThumbnailGenerator::CROP_OUTSIDE);
+			ThumbnailGenerator::CROP_OUTSIDE,
+			IImageManipulator::FORMAT_PNG);
 
 		$image = $imageManipulator->loadFromBuffer($result);
 		$this->assertEquals(150, $imageManipulator->getImageWidth($image));
@@ -62,7 +65,8 @@ final class ThumbnailGeneratorTest extends AbstractTestCase
 			$this->getTestFile('image.jpg'),
 			150,
 			150,
-			ThumbnailGenerator::CROP_OUTSIDE);
+			ThumbnailGenerator::CROP_OUTSIDE,
+			IImageManipulator::FORMAT_PNG);
 
 		$image = $imageManipulator->loadFromBuffer($result);
 		$this->assertEquals(150, $imageManipulator->getImageWidth($image));
@@ -72,7 +76,8 @@ final class ThumbnailGeneratorTest extends AbstractTestCase
 			$this->getTestFile('image.jpg'),
 			150,
 			150,
-			ThumbnailGenerator::CROP_INSIDE);
+			ThumbnailGenerator::CROP_INSIDE,
+			IImageManipulator::FORMAT_PNG);
 
 		$image = $imageManipulator->loadFromBuffer($result);
 		$this->assertEquals(150, $imageManipulator->getImageWidth($image));
@@ -89,7 +94,8 @@ final class ThumbnailGeneratorTest extends AbstractTestCase
 			$this->getTestFile('text.txt'),
 			150,
 			150,
-			ThumbnailGenerator::CROP_OUTSIDE);
+			ThumbnailGenerator::CROP_OUTSIDE,
+			IImageManipulator::FORMAT_PNG);
 	}
 
 	public function getImageManipulator()