Since version 1.6, SimplyTweet allows you to update your user profile. And in 2.1, you can also update your avatar/image from within SimplyTweet.
You happy try it out, excited that you can now conveniently change your avatar everyday to suit your mood.
It fails.
I've finally figured it out with the help and patience of several SimplyTweet users. Chances are you signed up with Twitter as a screen name with uppercase. Eg. "SimplyTweet" (instead of "simplytweet"). In that case, you need to log in exactly as "SimplyTweet" too. Otherwise, everything will work, loading timelines, posting, etc. The one thing that fails is updating your profile.
I've filed a bug with Twitter on this.
