Foto-Mode allows to create image snippets out of MuseScore scores. It can be toggled on/off with the Foto-Mode button.
In Foto-Mode, a selection rectangle can be spawned with Shift + mouse drag.
The selection rectangle can be moved with the mouse, or resized by moving one of the eight handles.
Once you specify the bounding box of the image snippet you want to create, right-click into the rectangle to popup the context menu:
Saving as a PNG file results in this file:
If you save your snippet in "print mode", it will appear as a cut out of the score as it would be printed. In "screenshot mode", the image will look like the score on your screen (including line break markers, etc), which are not printed (100dpi example):
See also Image to insert an image in your score.