Okay, we have about three issues here, and I'll attempt to answer them all.
Firstly, the login. There's not much I can say there. It may be operator error, especially since, if you're like most of us, you're typing your password in very quickly, which means spelling errors are possible. There are a few solutions for that. The first is, obviously, type your password very slowly, making sure to get the order correct. The second is to check the "Always Stay Logged In" box; that way it'll keep you logged in on here. The third I'll detail below, since it's associated with the second issue.
Your second issue is with the browser's password storage. That's an easy fix, thankfully: do not save anything until you get a confirmed login. The password save dialog should remain until you click Save or Don't Save, even when it changes pages, so it'll give you plenty of time to confirm the login and allow you to save the password. This also makes sorting out your login easier on that browser in the future, as you won't have to type everything again. (Just be a little careful with it, though; the autofill for passwords can be delayed until you click on the password box, then only autofill it then, so be conscious of that.)
The third issue is the avatar. I'm trying to find our size limits for avatars, but they're evading me currently. I normally recommend keeping it simple and under 600kB. Plus you can upload to the board directly, where it won't be affected by hotlinking issues.
Hope this helps in some way.