XnView Software

For completeness, did you check the image properties and confirm that the DPI X and Y values are identical?

Yes, the DPIs are the same. (I've never even heard of having different DPIs for the X and Y.)

Well, I did measure them and they're the same. :(
In trying to figure out why they look different to me, I changed the background color in both programs but that wasn't the problem. Finally, I put the two programs side by side with reduced windows and I found out that whichever image is on the left ...

I opened a TIFF file in XnView MP and thought it looked "longer" than normal. So I used "open with" to open the same file in my editor and the two images are quite different. In the example posted, it's very easy to see because the eyes are approximately at the same level but the chin in XnView MP ...

I got tired of messing with it and did a system restore to noon yesterday. (Luckily I knew that I had had 6 open before.) Now I can open as many instances as I want.

The system restore dialog didn't show any programs affected by the restore, so I don't know what caused the problem.

I've been using XnViewMP for a few months. I use it as the default viewer for tif and tiff files, which are mainly what I work with. Yesterday I tried to open a picture by double-clicking it in Windows explorer (which is what I always do), and nothing happened. So I right-clicked it and chose "open ...