I'd suggest using sprites. Sort of like this:
imgID = 1
load image "myImage.png", imgID, 1
sprID = 1
sprite sprID, 0, 0, imgID : hide sprite sprID : set sprite sprID 0, 1
...
alpha = <any value from 0 to 255> (255 meaning your image will appear the way it usually would and 0 being completely transparent (i.e. invisible))
set sprite alpha sprID, alpha
paste sprite sprID, x, y
..if that answers your question. To determine the alpha value, a timer system might indeed be a good option. Or in case of a fixed frame rate, just a value in/decreased by constant steps each cycle.
