I've made a collection of avatars for use on the forums (and others, if you so desire).
To add an avatar:
1. Right click the avatar you want, then go to Properties. Copy the address given.
2. Go to your Control Panel (the link can be found near the top of every page in the forums).
3. On the left side of the new page, go to Edit Avatar Settings underneath Personal Profile.
4. Paste the address of the avatar (found in step 1) into the URL field.
5. Click Update Avatar.
ALTERNATIVELY
1. Right click the avatar you want, then click "Save Target As..". Save the avatar image where ever you want.
2. Go to your Control Panel (the link can be found near the top of every page in the forums).
3. On the left side of the new page, go to Edit Avatar Settings underneath Personal Profile.
4. Click the Browse button. Open the avatar from where you saved it here.
5. Click Update Avatar.
If you'd like a custom avatar, please feel free to ask in this thread. I'll make it for you (so long as it's mod related).
As a side note, you may have noticed that the Spearman of the Havens is looking a little different in the avatar. I've decided to do the same with him as I did with the Riders: make unique armour rather than just a Galadhrim breastplate . The concept behind this armour is that the Spearmen are a memorial formation dedicated to Gil-Galad. As such, many of the motifs and ideas used with Gil-Galad's outfit are included here, with the addition of wave-shaped shoulder plates for the heavy armoured version.