Not sure if maybe the feature hasn't been really implemented yet, or if this is indeed a bug, but...

When I attempted to use the 'new thumbnail' option, (wanted something a little more representative of the current art style than the very first page of my comic) it allows you to browse for the file, but the 'send' button does not appear to be operational: the file texts writes right over it and it's not clickable. =/

That's very strange. When I tried changing a thumbnail to make the tutorial, after I browsed and chose the file, there was no change to the page (making me think at first that it hadn't remembered the file I'd chosen)... but when I cliked "send" the changes went through anyhow. I suppose I'll have to make another fake account and see what it looks like now...

I was using IE, if you are using Firefox try it in IE?

Maybe it's a firefox incompatibility. that's what I'm using, and all the send button is is a picture rather than an actual button.