The pngs with alpha transparency I imported and use as background images for the buttons all look like gifs with sharp, pixely edges. But I like my Buttonbackgrounds smooth 8)
Did you change the game's color depth to 32bit, then (re)import the pngs?
This seems to be a problem when the background of the GUI itself is also transparent in some way. Unfortunately there is as of yet no fix for it I am told (http://www.adventuregamestudio.co.uk/yabb/index.php?topic=43401.msg577180#msg577180).
I believe that the GUI background image has to have an alpha channel for the GUI buttons' alpha to work. So even if you don't need partial transparency in the background image, just make sure that it has some transparency somewhere and tell AGS to import it using the alpha channel.
thanks. Indeed adding a 1px x 1px transparent png as Background image for the complete guide made the GUI button background alphatransparent. But in the meantime I made the buttons even better and don't need this anymore. But good to know for future GUIs.