There are a handful of Image Creators / Photo Editors / Viewers in our
Freeware DB. I have not tried them all but I would assume all but the viewer have two solutions to your problem (and even IRFANVIEW has at least one of them).
1) crop, where it allows you to select a portion of the image and then you can 1a) paste selection to a new image or 1b) crop to selection, discarding the excess in the current image.
2) resize which allows you to shrink or enlarge your existing image to a specific pixel height and width. You may need a combination of the two to get as much picture within the limits as possible and still chop off an edge or two rather than distorting the image.
You will need to know what the pixel limits are for their site and then tweak you image to that size.
If you do not wish to learn a software package, as Ace said you could use the "Go advanced" button and then the paperclip option to attach your image here and tell us the pixel size needed and we could easily adjust it.