Which version of Android is it running?
You should be able to find that by going to settings / general / about device
My phone wouldn't post pix when I was running a version 4.x, but once it upgraded to 5.x it worked.
There is a lot I like with the interface of the new version of Android, but it also is slower. That might just be a change in the sensitivity of button pushes, I have to poke around settings and see.
It downloaded the upgrade on its own (I have it set to only download when it's on wifi, to save data charges). Then a message popped up saying the upgrade was ready to install, and giving me the option to install at that time or later (or to decline). I'm on AT&T
I hadn't tried this in a long time, I just tried, also Android 4.4.2, Samsung Galaxy S5, the Embed and Image/Upload from my device button doesn't work. No response to it at all.
By the way, my avatar instructions above are for the desktop version. I don't believe you can upload an avatar from the mobile version. However, if you scroll to the bottom of the mobile version there is a button to change to the desktop version, and that one should permit you to do the deed.
Click on "my profile" on the upper right of this screen. That brings you to your profile page, and on the left is your avatar or the generic placeholder image for an avatar. Underneath it you'll see a link that says "replace avatar"... click that. Then you can browse your computer or the...
It seems your browser isn't rendering the pages completely; maybe it doesn't allow for the javascript they use to make everything work on this site. Are you able to try with a different browser?
Are you using the native browser or a different one (such as Firefox)? It sounds like the browser isn't interpreting the popup image loader properly.
Check if there is an update available for the browser. I did upload today from the native browser on an S3, Android v. 4.1.1