I don't know how much room there is, but how about something like:
A screenshot is worth a thousand words. Help us see what you're seeing by posting a screenshot.
Here's how: [LINK]
And please PREVIEW the result before posting.
I'm not sure about this one.
While it might make sense in some cases to motivate a user to add a screenshot to a post, for most boards and new posts it isn't relevant or appropriate.
It might trigger many new posts needlessly dressed-up with images that don't really add any value.
Or worse, quite a few (new) members already seem to believe it's not necessary to write coherent sentences or explanations, thinking we are all mind readers here.
If they are stimulated to post screenshots, there is a good chance they won't bother to even explain what we should look for at all.
So I stick to my original suggestion.
Make it clear that IF a member wants to add a screenshot and doesn't know how to, point him to the tutorials.