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

Dungeon Channel

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