I could reproduce that it doesnt work if the image url contains a non-image-format.
For example php generated images like facebook.com/....safe_image.php?params... are not working.
The generated image looks like "filesystem:http://localhost:8100/temporary/imgcache/155705319ed7e5a266fca6805a3a24c2b1157e5b.php" which is of course not readable as image.
I could reproduce that it doesnt work if the image url contains a non-image-format.
For example php generated images like facebook.com/....safe_image.php?params... are not working.
The generated image looks like "filesystem:http://localhost:8100/temporary/imgcache/155705319ed7e5a266fca6805a3a24c2b1157e5b.php" which is of course not readable as image.