How to mask images?

In pre BlitzMax you ware able to mask images like MaskImage testImage, red, green, blue but this no longer works in BlitzMax. The only two commands that have something to do with masking are SetMaskColor and MaskPixmap. I don't even have a clue what SetMaskColor does since I don't see any changes whit SetMaskColor on or off. Then you have MaskPixmap... First I don't really understand the thing whit pixmaps... So when I want to change something of an image (in this case masking the image), I have to take the image out of VRam and put it into CPU addressable Ram, right? And this is done like:

Local pixmapTestImage:TPixmap = LockImage(testImage)

MaskPixmap pixmapTestImage, red, green, blue

UnlockImage(testImage)

But when I do this I get an Runtime Error: Image frame cannot be locked. Cane somebody help me out whit this?

Thank you, Nicolas.