Yeah - GIMP is great, and free.
Are they too big as in pixel dimensions, or file size?
I can't remember if there was a pixel limit on the free account, but I know on the paid account the only limit is that each individual file has to be less than 20MB (which is pretty big, for a JPG...).
edit
And if I remember correctly - if you try to upload something bigger than that, it will automatically resize it to reduce the file size. I used to upload a lot of 4800x7000 pixel film scans, and they were usually around 18MB, but sometimes over 20MB... I noticed that the larger ones were smaller (in pixels) once they were uploaded.