Question closed! → Just 15 min later I realized that the JPG format is only used when resizing!
This way I have implemented it in my old plugin as well :) good work gidgreen!
I disagree with the solution of always throwing back a JPEG.
With PNG you can save a lot of data and compress much better if it is an appropriate image. E.g. make a screenshot, you will notice PNG is much better in compressing it. Jpeg is better for real life photos.
I suggest changing the: function qa_image_constrain_data($imagedata, &$width, &$height, $maxwidth, $maxheight=null)
so that it returns a PNG image if the image uploaded was PNG. Code example:
$src = imagecreatefrompng($file['tmp_name']);
// create new image
$newImage = imagecreatetruecolor($newwidth,$newheight);
// do the image resizing by copying from the original into $newImage image
// write image to buffer and save in variable
ob_start(); // stdout --> buffer
imagejpeg($newImage,NULL,90); // do JPG instead of PNG
$newImageToSave = ob_get_contents(); // store stdout in $newImageToSave
ob_end_clean(); // clear buffer
// remove images from php buffer
$blobid=qa_db_blob_create($newImageToSave, $extension, @$file['name'], $userid, $cookieid, qa_remote_ip_address()); // v1.5.4