I have the Picasa photo viewer (PicasaPhotoViewer.exe) set up as my default app for basic image viewing. It's generally fast, makes it easy to scroll through a folder full of images, and lets me zoom in and out at lightning speed using the mouse wheel. All good.
Unfortunately, sometimes I would notice in my Task Manager window (this is on Windows 7 Pro) that I had a whole pile of PicasaPhotoViewer.exe processes still running, even with no photos left open onscreen. It was easy to terminate them (though with 30 processes a bit tedious) but it's always annoying when things like that happen, especially when you can't find an obvious answer.
As it turns out, the process may (or perhaps will) stay resident if you are loading files across the network. I have a server with a largish hard drive (RAID5) and keep lots of photos and such on it, so from time to time I browse a folder on the network. If I'm repeatedly viewing an image then closing the viewer, regenerating the image (say) at the command line, then reloading it, each iteration opens a new process and leaves the old one in memory.
So there's no fix, but at least I now have an answer and can stop worrying about it. It's just another of those little annoyances from Google, where if you could only just figure out how to get someone there to hear about, acknowledge, and fix your problem -- or do any one of those! -- life would be great.
Thanks "neilwales", whoever you are. :)
Edit: this is fixed in build 117.38 and later.