I am not sure where you got your information about finding posters on the internet in jpeg format, but I think you are going to find that a rather difficult find. First of all you don't print posters from JPEG files. That format is strictly meant to be viewed onscreen. The type of optimization it uses to trick your eyes into seeing a picture does not work very well in print. Expecially poster sized prints.
Which brings up the next issue, the typical 4 color process, print quality file is going to be somewhere in the neighborhood of 80 to 150 megabytes depending on your printer's specificiations. I don't think I would even -try- to load a file that large in Gimp, let alone the fact that it can't even handle the four color format, CMYK. Print is a very tricky business where sometimes just a 3 or 4 percentile error in one of your color channels will throw the whole thing off and give it a noticeable cast. So getting prints done involves tweaking colors, sending them to a proofer, getting the proofs back, making more tweaks, ect... Expecially if you have never color corrected for print before. Of course, maybe I am reading too much into this. Maybe you arn't looking for massive glossy full color prints. You should just be aware that if that is what you were thinking of, such things are going to cost a little money if you want them to look good. Unless you have a very experienced graphic artist amongst you. I'd be willing to give you a hand, but I don't think that my company would be excited about me running off proofs of Linux posters on the kodak Approval. Each run costs something like 10 bucks :) .:. Ioa Aqualine Petra'ka