I, myself, did have a little bit of trouble with this in the beginning and I remember being really confused as to why it wasn't working. Anyway, for all those wanting to know how to upload your in-game sim to use him/her as the avatar on the forums, this is how to do it.
To start with you're going to want to take your Sim to a mirror. It doesn't matter what clothes you have chosen to wear for the picture, your avatar will always be with your Sims 'everyday' outfit.
Once you've brought him/her to a mirror you will want to select it and click on 'Change Appearance'
Once this option has been selected you will enter the CAS [
Create
A Sim] mode
Now you can change your hair and makeup if you would like to but if you're happy with the way your Sim is you can simply click on the 'Share Sim' button.
You will then be prompted to enter a Name and Description for your Sim. You only need to enter a name for your Sim, but you can enter a description if you like. But it's not necessary.
Entering a name for this Sim does not effect the game at all. The name is simply for the purpose of the forum. For instance, if you have uploaded multiple images of your Sim you can have alternate 'versions' if you like.
Once you are done, click the big 'Tick' and the Sim will be uploaded to The Sims 3 Launcher.
When it has finished uploading, this dialog box will appear, simply click 'Accept'
Now you can either click the tick or cross depending on whether you want to save any changes you have made to your Sim while you where in CAS mode.
Once you're back in 'Live' mode, click on the menu in the bottom left hand corner, and click on Open Launcher. If you have a slower computer, you should probably just close The Sims 3 all together using the 'Save and Quit' or 'Quit' options and relaunch The Sims 3 launcher.
Now that you're at the launcher window, tick the pictures you would like to upload to the forum. Then click 'Upload'.
When your images have finished uploading, go to The Sims 3 website. It doesn't matter what part of the site you are at, the home page or the forum, anywhere. Click your name. It will always be near the top left hand corner of the website.
Now click on 'Edit Avatar'
Now as you can see, your uploaded Avatars are sitting nicely in a row on this page. Select the one you would like to use.
Then click submit.
And now you're done. Your own personal Sim to represent you in the forums as well as in game =]
I hope this was an easy enough tutorial to follow. Hopefully we will see more custom avatars in the forums now =]
Just a side note too. I'm not sure if there is any specific way to get a pose you would like your Sim to use for your avatar. I haven't done any testing with it. But as you can see there are at least two different poses for the male Sims. AS far as I know there is no way to get any specific pose.