For most questions, what we (and sites like stackoverflow.com) need is *text*, not pictures. Text containing log snippets, error messages, code etc. that we can work with to copy it into our answers (or maybe even test ourself). 

Screenshots don't work for this and makes it less likely someone will try to help you if you post a screenshot instead of copied/pasted text from your terminal. 

Naturally, this doesn't apply to everything and if a screenshot or other illustraton is the only possible way to show something, it's perfectly fine to post one.