floleb7 - asking users to make their own thumbnails would indeed be an answer to the problem, but how that works in reality

Well I just know for a fact that some people might concider that as a waste of time or just a pain in the ass, especially when they know the server could do it for them
The proof is out there. A lot of sites are using some type of script to generate their thumbnails, so it's just a question of finding one that works for everybody. I'm waiting for an answer from a few people to see what they're using to generate their tumbnails. Hopefully they'll be able to give us a hand...
etones - image magick ?