One of my colleagues, recently opened a bug and just attached a screenshot without any description or explanation, simply thinking that picture will explain everything. Here is why it didn't work:
1) There is no "steps to reproduce", developer might not understand how you got to that point.
2) Also, it doesn't tell the developer what is the expected result.
We currently use "Youtrack" to track all of our bugs/enhancements and it provides an end user with the great ways to attache a screen shot and I believe that it is IMPORTANT to add a screenshot, but picture by itself is not enough, at the same time you should not be writing a bug which has 1000 words :)
