Try this:
rem For testing
set window on
backdrop off : rem <------
rem HERE is the problem
load image "c:\test1.png", 1
load image "c:\test2.gif", 2
paste image 1, 0, 0
paste image 2, 150, 0
wait key
Or if you prefer manual syncing:
rem For testing
set window on
sync on : rem <------
rem HERE is the problem
load image "c:\test1.png", 1
load image "c:\test2.gif", 2
paste image 1, 0, 0
paste image 2, 150, 0
sync : rem <------
rem NOTE: Might need to sync twice because DBP is double-buffered
wait key
Unrelated, but you should use relative paths when loading media. Instead of "c:\test1.png" you should use "test1.png" and place the image in the same folder as your executable. Or if you want to be more tidy, make a folder in your project folder "media/images" and place them in there, and load it with "media/images/test1.png"