the difference between a TImage and a TPixmap is that a TImage is in video memory and a TPixmap is in main memory.
Actually a when you draw a pixmap the pixels are blitted to the backbuffer. When you draw an image BMAX creates a textured Quad.
Also, if you are going to use a large image to contain all tiles then do not use the inbuilt anim image commands. Bmax will create a new texture from each of the image frames - THIS IS BAD. BMAX will switch textures/surfaces everytime you draw a different frame THIS IS CRAZY.
You need one of these > http://www.blitzbasic.com/Community/posts.php?topic=51647
The above code will save you memory and increase the speed of your render.