I could have sworn I saw this same question answered somewhere else (I must be going crazy...because I don't see it now) 🤪
Anyway: First you need to copy the .bmp file into your AVS directory-the default is C:\Program Files\Winamp\Plugins\AVS
Then you need to add a Render-->Picture. Look for the name of the .bmp file you copied to the AVS directory & select it.
You'll have to do some tinkering to get the picture to not mess up your preset, though.
Hope this helps.
***Edit: Grrrr! I'm not as crazy as I thought...I found the question in the main AVS forum!***