Fixed an issue in which the avatar image falls down the qa-login-bar

I've provided a fix for an issue in this pull request: https://github.com/q2a/question2answer/pull/13

However, that created another issue that is explained here: http://www.question2answer.org/qa/32810. It seems images exceed the 24px size.

So I thought that a better approach would be just to rely on the sizes provided by the core and not change anything in the CSS. I tested this on Chrome 35, IE 11 and FF 29 and it seems to be working fine.