I have the Philips 7" photo frame and use wdpv from
http://www.willus.com/archive/ with the parameters -sort shuffle -crop -p -geom 720x480
This re-sizes the photo down to 720x480, but also crops the photo if required (will cut off a bit of the top and bottom). From what I remember if you leave the -crop bit off you get all of the original photo, but a black bar at either side to get it to the 3x2 ratio.
The other great thing about this is that it does a batch of photos (up to about 25 I think). I have a shortcut with the above parameters, and just select the photos I want and drag and drop them onto the short cut.
Ian.