comparing xfce4.6's panel to xfce 4.8's i noticed that the latter scales buttons down to a square shape and adds a 2px inline border. in xfce 4.6 you could do rectangular (or: wide) menu buttons, which would make them stand out, in xfce 4.8 the wide/rectangular images are reduced to the width of the panel's height (meaning: if the panel is 24px high and the image is a size of 24x48 it will get scaled down to 12x24). additionally there's the 2px padding, which further reduces theming of the menu button. keeping my fingers crossed that this is not too hard to fix :)
Fixed in de5e70d, for non-labelled buttons.
I'm still having the same issue in XFCE 4.10
Just tested it to be sure and it works for me in 4.10.