Unfortunately Combine does NOT support PSD files conversion. But you also mentioned that you are getting the same behavior with JPGs. Can you please provide me the JPG files you used to let me investigate the issue.
Also, can you try combining some other supported files (other than the ones you used) in combine (.jpg, .png , .bmp etc.) , to make sure that its not a generic problem with your installation.
Which Mac OS version are you using ? and did combine ever work on your machine or is it that you are using combine for the first time ?
Also, can you send me the files you used for this operation, I'll try them on my system.