That FastStone is a pretty nice program.....Find a picture of clouds, apply a bump map and make it slightly green.....and voila, you have old beatup green plaster wall.....in two clicks!
Here are some more program suggestions:
The Gimp - a very powerful freeware program that supports all image formats, along with TONS of filters...all free!
http://www.gimp.org/Corel Paint Shop Pro - a professional quality paint/photo program that's much more affordable than photoshop.
http://www.jasc.comAnd one secret tip for all up ppz: get a bevel filter. I dunno bout mspaint, but the Gimp has several. Using the BEVEL makes flat images look very 3d!