After the last viewer window issue I reported, I suppose this one might have the same root cause, too, but I have no way of knowing if that's true.
The problem is, if you open a very small image on Win7, the viewer window can be small enough that the Minimize button on the caption bar will overlap the window icon in the upper left corner. When this happens, the icon is painted on top of the minimize button. I'm not sure if that's actually how the system paints it, or if it's a bug in the software used by ThumbsPlus to handle viewer windows. Either way, this should be relatively easy to fix, by making sure the viewer window never gets smaller than the sum of the following:
1. Window icon size (usually 16 pixels)
2. Caption bar button widths (can be queried from OS)
3. Window padding (queried from OS)
4. Any other miscellaneous widths like window border.
Or alternatively, ensure the window's client size is never narrower than the sum of #1 and #2.