I don't have much experience with live mail, so I can't address that possibility.
We did some testing once with Adobe because the shortcuts for .pdf files were not showing as thumbnails. I believe it was the x64 Windows 7 version, but not sure, the thumbnails would only be created when you opened a folder with those files in it. Maybe in your case, the same type of thing is going on, except not thumbnails but a compressed version of the photo for other uses, as you suggest. If not Live Mail, possibly some other photo editing or viewing software. Since I have not seen the behavior on my system, I could only guess.
You might check any sharing settings in case a multimedia utility is getting them ready to share or stream, like a slide show. If you check the properties for the folder, or the parent folder, does it say anything about what the folder is optimized for? I suppose if it is optimized for photos, you would see small copies of the pictures in the folder icon.
I have been trying to think of something you could watch to see if any activity showed up, and all I might suggest is to watch Task Manager to see if the CPU usage shows on one of the Processes. If you were to notice a process becoming active at that time, we could pin it down.