Shrinking a file

This may not be the proper place to post this and it may be impossible to do but since I am computer illiterate I might as well ask. Some friends took a bunch of pictures at our first triathlon and put them on a cd and i brought it to work hoping to put one up on my navitar but it said that the file was to big. I asked some co-workers how to shrink it and of course nobody had any ideas. Can this be done? and if so how? Thanks much for taking your time to respond.