Regardless of your photo-editing software of choice, here are some links that you can use for map creation.

Textures

http://www.texturemate.com/category/image-galleries/asphalt

Paint Brushes

http://www.obsidiandawn.com/category/brushes

Tokens & such

http://www.rpgmapshare.com/index.php?q=gallery&g2_itemId=564&g2_page=24

http://rpgvirtualtabletop.wikidot.com/tokens

Predone maps

http://rpgmapshare.com/index.php?q=gallery&g2_itemId=19

Map Making Online

Here are some online map making tools that you can use to make maps for Roll20.

Dave's Mapper

AnaMap

Dungeon Painter Online

Tiamat The Tile Mapper