I can think of two approches you can explore:
1) use operating system settings to specify photoshop as the default application that opens this file type when it is double clicked. Then Export field contents with the option specified to automatically open the exported file will export the file and it'll open in Photoshop
2) Export the file without opening it, then use send event to open it with Photoshop.
1. The images are .jpg so changing the file association would mean any .jpg on the computer would open in Photoshop when double clicked and that would be bad.
2. How would I use send event and specify Photoshop? Would that require changing the file association as well?
Hmmm, I thought there was a way to use Send Event to both open a document and specify the application used to do so, but a review of FileMaker help indicates that it will open the specified file with the associated file--so we'll have the same issue here. (And there are distinct platform differences for send event as this sends an 'apple event' on mac systems.)
Looks like you'd have to set up and execute a system script to open the document with a specified application. In theory, that should be possible, but the exact syntax is not something I know without researching it and will also depend on whether you have a Mac or Windows platform.