There's no danger here, Google is just freaking out about Dropbox for some reason. Might be worth suggesting that Eversmile uploads his avatar to these boards rather than hotlinking though.
I also get a warning in Firefox 3 but only when I try to open the image alone. I have no problem looking at the thread page with the avatar.
I guess he should upload his avatar to the site, seems like the easiest solution.
You can safely ignore this warning. It nicely shows the stupidity of such blacklists. dropbox.com is a file hoster. Apparently someone hosted malware there and now Google says "everything at dropbox.com is dangerous".