If you are using OpenGL, you need to use a texture format that supports transparency layers, something like png and look up blending ( nehe lesson 8 - http://nehe.gamedev.net/data/lessons...n.asp?lesson=8 ). If you are just just looking for a color key type effect then you need to look up masking ( nehe lesson 20 - http://nehe.gamedev.net/data/lessons....asp?lesson=20 )

I hope this helps.