The picture should be 720×480, never have DVDSP resize a picture.
If all you want is this one image I would resize it, bring it into motion, make it a 5 minute movie, export it as 29.97 progressive Uncompressed 10 bit movie. Use compressor and make a CBR preset at 7 Mbps and 29.97 Progressive. bring this into DVDSP and loop it on the first play video track. (not a menu track) this should give you a good image.