attaching photos is a perk of membership in the planted tank. cost is $10 annually and you get a pm mailbox of 250 messages instead of... what is it, 25? and the ability to post images directly on to the planted tank server. otherwise, you'll need an image host (your own site, or a site specialized in photo hosting) i recommend www.imageshack.com
, but there are a few others. Since joining the planted tank I think I use the attach photo feature about 50% of the time, and the rest of the time I still use imageshack because it allows a thumbnail preview or directly inserting the image into the body of the message (plantedtank.net puts a link to the image in the body of the post). Both are acceptable means of sharing your photos with others, but sometimes the preview or entire image in the message is nice.