This booru (like many other sites) uses a service called 'Gravatar' for managing avatars. If you visit your profile and don't already have an avatar setup, you'll see a paragraph explaining that you'd need to crate an account with Gravatar (using the same email address that you used to create your account here) and upload your avatar of choice there. Once you've done that, the booru will automatically start using your new avatar.

How can I edit my comments?

Sorry, there's no way of editing a comment once you've posted it. Your best bet is to re-read the comment carefully before posting it. If you posted something that you really regret for whatever reason, you can always contact a moderator and ask them to remove it for you.

How can I send a message to someone else here?

Private messages (or 'PM's') can be sent to other users by visiting their userpage (by clicking on their name) where you'll then find an email-like form at the bottom of the page to fill in and send off.

Can I upload a new version of a old picture?

If your new version is a straightforward upgrade to the old version, we'd prefer that you didn't, otherwise the gallery would end up filled with near duplicates and iterations of the same pictures. Instead, please use the 'Report Image' form in the sidebar of your old image to request it being replaced with the new image (Don't forget to include a link to the new image).

Why does my 'artist tag' show up in with the normal tags?

It's probably because you misspelled or miscapitalized the 'artist' part at the beginning. It's case sensative, so typing it as anything other than all lower case will cause the booru to recognise it as a normal tag (and once a tag's been used for the first time, the booru will auto correct any future uses to use the same capitolisation, which can be a problem for instances like this). Don't worry though! An admin can fix that for you in a snap. Just send one a PM, or report the image that has the broken tag on it, stating that your tag isn't working.