I had this problem too. After playing with it a little bit and taking a couple more pictures, I think it only recognizes pictures that are on the internal storage. Try copying a couple pictures from your SD card to the internal storage and see what happens.
I have been taking pictures but the Photoshop still can't find the images....
Sent from my SCH-I535 using Android Central Forums
Are you saving new pictures to the external SD card or to the internal storage? That made a difference for me. Pictures saved to the internal storage were found by Photoshop Express or whatever it's called.