If you are framing them, foamcore is fine, but I would recommend 3/16 instead of 1/8. Some frames, especially inexpensive ready-mades may not have enough rabbet depth to support that. If you are framing them, then you don’t necessarily need to have them professionally dry or cold mounted. You can consider less expensive and more archival t-hinging depending on the environment they will be displayed. If you are going to leave them unframed then you need something that is stronger and more rigid than foamcore such as gatorboard. I have created a web page with lots of info about
mounting techniques here.
Cheers,
Mark