trekcore.com has a really great site of screencaps of all the Trek series and movies that you can download and print out. That's where I get my pics for autographs.
I recommend if you're going to print your own photos, that you select light colored images. If you pick a dark image the actors use a silver marker and the silver rubs off easily if you're not careful.
Any time I've ever tried to print from DVD screen caps, the result is pretty grainy and blurry. Only the blu-rays caps have recently been high resolution enough for such, IME. Do you have some sort of special program or up-scaling?
I do like to bring my own photos though, and have printed some up. But I usually use high resolution photos for this. The only successful screen-capture autograph I've yet done is from a blu-ray cap. The rest where from high resolution photos (I'm talking high res as in 1800x2600 pixels area).
There are also one or two places you can buy from. The only two I know of off the top of my head are statesidestills.com and celebrity8x10s.com - and one of them (can't remember which!) I've found their quality wanting frankly...
An example some of the photos I've had prints made of from on-line resources (though my scans of them don't do them justice, lol
Locutus of Borg
- Sarah Connor/Kyle Reese
Data and Tasha Yar
Kirk and Spock
Janeway and Tuvok (of Borg)
Kirk and Picard
Data and Geordi
- this is a blu-ray screen cap. Levar Burton liked it so much he DM'd me on Twitter to get the original file.
Of all of them only the one is a screen cap (from blu-ray) and the rest are high res photos acquired off the web. They look much better in person than my scans, lol.