You could reverse the UV using SetUV which would be pretty quick. It seems the BlitzMax way is to use XFlipPixmap and YFlipPixmap. Finally, the easy way, you can setscale -1.0,1.0 to flip around X but it moves the imagehandle as well (which you can simply move back with setimagehandle)
Graphics 800 , 600 Local image:TImage = LoadImage("max.png") 'SetImageHandle (image,ImageWidth(image),0) While Not KeyHit(key_escape) Cls SetScale -1.0 , 1.0 DrawImage image , 200 , 200 Flip Wend