modifying bitmap background

Im confused about this. I only need to create a single bitmap into background, but I cant do it. Whyy!! what I need to do!

I need something like this:

bm1 = bitmap 60 60 color:red
pasteBitmap bm1 background (box2 0 0 60 60) [50,50]

PS: The example is only ilustrative. It is not my real problem.