Ok, I noticed an ebuild in the help page for the imagery plugin as well - but it still has the dodoc entry that will not work (since README is gone in svn)
Next thing - while webkit-image compiled fine and downloads stuff from yahoo it does not assemble the tiles in josm properly. I can see the image in a square, but it seems to be smaller than the space reserved for it. It aligns top left and does not fill the gray area square reserved for it. Also, the images do not align to the data (I'm not talking about the 5-10m offset due to yahoo inaccuracy - the offset is huge compared to that..)
Greetings..