diff --git a/apps/theming/lib/ImageManager.php b/apps/theming/lib/ImageManager.php
index 6f9892af10e38..7c3d4a2e4d684 100644
--- a/apps/theming/lib/ImageManager.php
+++ b/apps/theming/lib/ImageManager.php
@@ -232,29 +232,67 @@ public function updateImage(string $key, string $tmpFile): string {
 		}
 
 		if ($key === 'background' && strpos($detectedMimeType, 'image/svg') === false && strpos($detectedMimeType, 'image/gif') === false) {
-			// Optimize the image since some people may upload images that will be
-			// either to big or are not progressive rendering.
-			$newImage = @imagecreatefromstring(file_get_contents($tmpFile));
+			$tmpFile = $this->getResizedFilePath($tmpFile, $detectedMimeType);
+		}
 
-			// Preserve transparency
-			imagesavealpha($newImage, true);
-			imagealphablending($newImage, true);
+		$target->putContent(file_get_contents($tmpFile));
+		return $detectedMimeType;
+	}
 
-			$tmpFile = $this->tempManager->getTemporaryFile();
-			$newWidth = (int)(imagesx($newImage) < 4096 ? imagesx($newImage) : 4096);
-			$newHeight = (int)(imagesy($newImage) / (imagesx($newImage) / $newWidth));
-			$outputImage = imagescale($newImage, $newWidth, $newHeight);
+	/**
+	 * For png and webp files: Returns the file path of the file resized to a maximum of 4096 pixels wide (preserving aspect ratio)
+	 * but only if the resizing results in an image that has a smaller file size than the uploaded file. For jpeg files: 
+	 * Will always re-save the file at quality 80 (not too low, not too high to create large files).
+	 *
+	 * @param string $originalTmpFile The tmpFile(path) as uploaded by the user
+	 * @param string $detectedMimeType The the MIME type of the file as uploaded by the user
+	 * @return string Location of the resized file, or the original if that was the smaller of the two
+	 */
+	private function getResizedFilePath(string $originalTmpFile, string $detectedMimeType): string
+	{
+		// Optimize the image since some people may upload images that will be
+		// either to big or are not progressive rendering.
+		$newImage = @imagecreatefromstring(file_get_contents($originalTmpFile));
 
-			imageinterlace($outputImage, 1);
-			imagepng($outputImage, $tmpFile, 8);
-			imagedestroy($outputImage);
+		// Preserve transparency
+		imagesavealpha($newImage, true);
+		imagealphablending($newImage, true);
 
-			$target->putContent(file_get_contents($tmpFile));
-		} else {
-			$target->putContent(file_get_contents($tmpFile));
+		$newImageTmpFile = $this->tempManager->getTemporaryFile();
+		$newWidth = (int)(imagesx($newImage) < 4096 ? imagesx($newImage) : 4096);
+		$newHeight = (int)(imagesy($newImage) / (imagesx($newImage) / $newWidth));
+		$outputImage = imagescale($newImage, $newWidth, $newHeight);
+
+		imageinterlace($outputImage, 1);
+
+		$checkIfFileSizeWasReduced = false;
+
+		// Preserve the original file format
+		switch($detectedMimeType) {
+			case 'image/png':
+				imagepng($outputImage, $newImageTmpFile, 8);
+				$checkIfFileSizeWasReduced = true;
+				break;
+			case 'image/webp':
+				imagewebp($outputImage, $newImageTmpFile, 80);
+				$checkIfFileSizeWasReduced = true;
+				break;
+			case 'image/jpeg':
+				imagejpeg($outputImage, $newImageTmpFile, 80);
+				// we have no convenient way if knowing if the original file was a progressive
+				// jpeg, so we just need to re-save it. 
+				$checkIfFileSizeWasReduced = false;
+				break;
 		}
 
-		return $detectedMimeType;
+		imagedestroy($outputImage);
+		
+		$newImageIsLarger = filesize($newImageTmpFile) >= filesize($originalTmpFile);
+		if($checkIfFileSizeWasReduced && $newImageIsLarger) {
+			return $originalTmpFile;
+		}	
+		
+		return $newImageTmpFile;	
 	}
 
 	/**