For the record, there was no need to resize your album art manually. Rockbox is quite capable of downscaling large images. If the art displays in the while playing screen or base skin, it would've worked fine for pictureflow as well.
However, in saying that, any album art larger than the smallest screen dimension is a complete waste of space. Embedded art even more so, by an order of magnitude equal to the number of files present per album (needless duplication).
Regarding the menu, it is quite likely that the keymap is incomplete. Plugins aren't usually very high on the list of things to get working on a new port.
I am glad you got it working, though.
[Saint]