Size_x and Size_y will be set by the system so no need to put that in unless you are using Windows with no bmaps in the panel itself. PNGs are very happy in panels, I use them all the time.

When normal things don't fix your problems try abnormal things. Delete teh panel and make another panel with a different name but same bmap. If it still doesn't work try a different bmap but same size. Now if it doesn't work try altering the first bmap. If that doesn't work then it is something unrelated but only seems to be the panel. A problem I have had before especially when projects get huge.