Thanks for the replies. Yes, I agree with Ingolme : Photoshop is very expensive and not much people are familiar with Gimp (and IMHO, it is not very intuitive). Both are too big and slow tools to efficiently do the small editions we need to do over many large images.
We need to go over millions of images, spending about 15 secs on each of them. This is very tedious and we need a good tailored tool to avoid any inefficiencies. We need to be able to train staff quickly, no need to learn these complicated commercial softwares and all their options.
We keep an opened mind but it will definitely be a web app. Any other pointers to existing libraries, tools, applications,... ?
Thanks!