You should make the maps as ASCII files rather than bitmap especially since you want a 2d grid based map. Bitmaps are good for creating heightmaps for 3d terrains but this is not what you want.
See this tutorial:
http://files.thegamecreators.com/developer/dbpro/DBPro_Tutorial_6_Huge_Dungeons.zip
It's for a 3d map but the concepts are the same so it wouldn't be difficult to convert it to 2d. Just program it so it makes a 2d world based on the map file.
For trees which are more than one tile big, you will have to split the tree into different sections and build up a tree in the game by piecing together all the bits on the map.
say a tree is 6 squares big it will look like this:
1 2
3 4
5 6
Basically each symbol loads a different tile. You'll understand when you read the example and have a look at how maps are made in this way.
This is pretty much how maps are made in Zelda and a tile based level will give your game more of a classic feel to it as well as making level designing much easier. It would be much more easier to make a map maker for your game if you wished. Also bitmaps use up much more space than ascii files and would take longer to load.
