The easiest way I could think to do this is to just take a screenshot of it. If you are on windows, you can
1. Select window and press Alt+PrintScreen
2. Open Paint (or some other image program) and copy the clipboard contents to a new image, then save it as a JPG.
If you need to automate the process you could probably do it by finding the handle to the SimMechanics visualization window and then printing it.
>> allFigs = findall(0,'Type','figure')
>> simMechFig = allFigs(2);
I'd stick with the first approach, though.