Sorry for the lag -- the team has been away on a winter break.
I have been able to open large 10 megapixel images. What is your image resolution?
Users have reported intermittent issues opening and editing images of any size, and doing a full backup/restore of the device has helps. Before trying that, you might instead see if you can open the image after syncing it to your device via iTunes. iTunes downsamples large images so they take less storage space and work easier on iPhones and iPads. This could also help you get your image working in Photoshop Express, though it will not be your original.
Hope this helps!
Please try the latest Photoshop Express version 3.1.1 available on appstore. this should support the image size required by and solve your problem.