Hi folks

Sorry for the following noob question: How do I extract tiles from a tileset such as this one:
http://opengameart.org/sites/default/files/grassland_tiles.png

I want to use these tiles in my game. I see that some tiles here are of different sizes (or maybe all tiles are a multiple of some fixed size). If I want to use one of the tree tiles in my program, how can I figure out what offset into the tileset I must use, and how big of an image i must extract?

Thanks!