Is it allowed to use images to paste math formula in mathoverflow.net ?
I think this is high efficiency, and few people search math questions by LaTeX.
I stumbled upon this post on Meta Stack Exchange, where various reasons why typing the math formula is preferred over including images: Why are images of text, code and mathematical expressions discouraged? (There are several rather detailed answers - I recommend to anybody interested in this topic at least having a look there.)
Let me summarize briefly at least some of the reasons given there:
Having said that, sometimes adding a picture with some page from a relevant text might be useful. But some effort should be made to make the question understandable without the picture - and consider the picture just as a possible source of an additional information.