Depending on how the PDF file was done, you can use the select tool and click on the picture you want. If you see it get selected (in negative) then you can copy it to the clipboard and paste it in any picture editing software and save it from there. Sometimes the select tool will grab the whole page, so you'll have to crop the picture before saving.
Beware that some PDF files make such manipulation of pictures very difficult because once pasted, the pic will be distorted. In that case it's best to do as was previously recommended: zoom in and capture the screen.
Hope this helps!