There are 3 ways you can post pictures on this site:
Hosting directly on this site:
1) If you only want to post your picture in a single post you can simply add the picture as an attachment whilst you are making your post
Click the "Manage Attachments" button below where you are typing your post or click the icon.
If you can't see the "Manage Attachments" button you are probably in quick-reply, in which case click the "Go Advanced" button. The "Manage Attachments" buttons should now be visible.
2) If you want your photo to be visible in the Gallery and in Posts (or even on other sites if you wish) then add it to your gallery instead. Click the Gallery menu in the orange menu bar, then click "Upload Photos" (over on the right hand side). You can then copy and paste the [img]http://.....[/img] bit into your post to show the picture.
3) If you have a photobucket/flickr account or similar you can upload your photos there and paste the [img]http://.....[/img] bit into your post to show the picture.
Click to enlarge