In later version of PowerPoint, right click on it and go Save as Picture. In
earlier version, save as a web page to a folder on your hard drive. Then go
looking for the image in that sub-folders of where you saved it (make sure
you save as html and not mht).
If you only have the viewer, not many options there.
If you want to save the whole slide as a picture you will have the save as
option. Its just a little hidden! Choose save as and the in the box "save as
type" SCROLL DOWN to jpeg. If its just one picture then do as Glen says.
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