A lot depends on what images you intend to process.
Size, rate, type, encoding, hardware interface, storage,
display, network links, control panel, reliability etc.
My guess is that the BeagleBoards have more working peripheral
stuff surrounding them. If there is enough memory and
processing power that is an edge. The PandaBd has more
cores and more RAM... You can build a side by side
table and validate the facts both are well documented and
open designs.
Image processing is difficult with any platform because codecs are
not all public, data is often big, hardware (cameras and displays) can
be expensive and difficult to interface. DC Power limits on both Beagle and
Panda boards confuse many new arrivals.
Have you tried to prototype your goals on a laptop yet?
This is from the beagleboard site and applies to both Panda and Beagle boards
to one degree or another: