static screen shots

I'm about to insert some static screen shots into a Storyline presentation. In Articulate Presenter there was a technique of having screen shots at a particular size depending on what template you were inserting them in to (tabs, f.a.q's etc.). 

Do screen shots need to be of a particular size for Storyline to get the best quality? and do they need to be saved as png's or can they simply be 'snagged' and copied and pasted in?

Cheers

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