If you're using a PC, open it in Paint (under accessories), then save a copy. (You may want the original size at some point, so I always recommend resizing a copy.) I don't remember the exact path (I'm working on a Mac right now and don't have a PC handy), but you can scale the picture and stuff. The KEY is to change the pixel size, which is done by adjusting the resolution. Somewhere in there is a box to change it from, say, 300 ppi to something else. Because Paint is unsophisticated, you'll need to do it in multiple steps, such as resize then save, change the resolution then save, that kind of thing.