Hey sama. My bad. The original issue is not exactly a flickering. It is just the image not having the space it will take reserved before it appears. This results in the image not taking any space and suddenly appearing and pushing everything to the right. I guess you could reproduce this in qa 1.6.2 because I submitted the pull request by the end of december.
So to answer your questions I have to resort to my memory:
Q1: No. I think I found it on Chrome and FF as well.
Q2: From the 3 themes provided in the default install Snow is the only one showing the avatar in the login bar. So I'd answer with a Yes.
Q3: Good question. Yes. The other ones do not get that class qa-logged-in-avatar applied.
The core handles the size properly on its own by setting it to the image itself. I don't think there is any need of changing/overriding that in the theme. That is why it works on the rest of the avatars: they are not affected by this style.
I made a quick test on IE 11, FF 29 and Chrome 35. Both issues seem to be fixed with this. I've already sent the pull request with this change for Scott/Gideon to also test it. Although I have already tagged Jatin in the pull request, I guess he will not keep maintaining Snow v1.3 and, IMO, Q2A v1.7 will be released with Snow 1.4.
If we also take into account that, against my wishes, Q2A 1.6.4 will never be released, it makes sense to conclude that submitting this to github could be pointless but still I had that personal debt to clear :)