There's a nice and FREE imaging program a bit more functional than MS Paint called Irfanview. I use it pretty often. It allows you to change the size of pics or even save them as a different format. Play around with it a little and you'll get the feel for it.
My digitla camera takes pics at 300dpi and about 1600 resolution. I have to go into Photoshop and change it to 100dpi and 500 resolution before I can post a picture. You have to find a program that will let you change those properties. There are several out there. The ones listed above will work just fine. In Photoshop its under IMAGE and IMAGE SIZE