You have to reach 50 posts or make a donation (become a GT supporter) to get an avatar, but you can post pics in a thread at any time. Do you have a Flickr or Photobucket account? They both have a code you copy and paste (outdated but still useful Photobucket instructions are here). If you have a Facebook account, that's even easier: just right-click on the photo, select Copy Image Location (the exact wording may vary a bit depending on browser/OS), come back to your Greytalk post, click the little icon of a Polaroid above where you're typing the message, and paste the copied link.