If we need to show a screenshot (pertaining to error messages for example) in comments, what should we do? Is it worth allowing us to upload images to the comments?
Comments are supposed to be small remarks on question and answers -- they feature a limitation of 600 characters. If you feel the need to add a screenshot to a comment, you a) are in all likelihood the person who asked the original question b) should edit your question to include the additional information (and the screenshot). If you are not the questioner and by all means need to add a screenshot, ask a new (follow-up) question with a link to the original one.
There were a few instances where I did use the "Ask Question" page to upload an image that I then linked to in a comment, using the markdown syntax
[this is some text](http://address-of-comment). But in general I agree with what lockstep says.