What kind of effect are you trying to accomplish? Without knowing that, it's hard to give specific pointers.
You may find the following of use, something I put in the code archives last year: http://blitzbasic.com/codearcs/codearcs.php?code=1281
Fade to black, fade to white, and a crossfade between two images.
And another one I did, that's somewhat related: http://blitzbasic.com/codearcs/codearcs.php?code=1282
Fade an image from color to black-and-white, and back again.
(Note: these were written quite a while ago. You'll need to remove the 'flushmem' command from the linked sources, it is no longer necessary/supported.)
You may find the following of use, something I put in the code archives last year: http://blitzbasic.com/codearcs/codearcs.php?code=1281
Fade to black, fade to white, and a crossfade between two images.
And another one I did, that's somewhat related: http://blitzbasic.com/codearcs/codearcs.php?code=1282
Fade an image from color to black-and-white, and back again.
(Note: these were written quite a while ago. You'll need to remove the 'flushmem' command from the linked sources, it is no longer necessary/supported.)