"Action cannot be performed because this file is not modifiable"
This error message indicates that a FileMaker file either has "read only" status specified or is in a directory for which the current user account (on the OS, not FileMaker ) does not permit modifying the file. Best bet is to check the security settings on the folder(s) that contain this file and also to check and see if you need to repair user account permissions. (Details depend on what OS you have.)
Since you are being asked by FileMaker for an account you don't see in Manage | Accounts and Privileges, it's possible that you have an additional fileMaker file with different access accounts than the file you see open before you. If so, you may need to open Manage | External datasources and see what other files might be linked to this one and check their accounts and privileges to see if they might be the source of the open file dialog. I strongly suspect that's the case here as you clearly have some files already open when this dialog pops up.
Thanks, that was fast. I think you're pretty much right about everything. I opened up Manage | External datasources and I see another database where I think the images were being saved. When I tried to open that remote database of where the images are, I was able to see the images of previously uploaded images but I wasn't able to change anything (I didn't have the privilege). I think I'm going to have wait to fix this because I dont have access to the server, but thanks again.