This PHPBB2 forum software doesn't have a picture hosting service built in (yet). It's a canned product, and I haven't looked over the 500 mods available -- I have a life outside of this place!
If you already have your picture hosted somewhere (like your own webspace), you use the [img] tag to insert the URL to the picture, and when you post, the picture is shown from whereever it's hosted.
If you don't have your picture hosted somewhere, you can use a free photo hosting service and try to link to it -- doesn't always work, as the free services want you to see their ads, and won't let links work.
And, if you really can't upload your pics to your server (your ISP's server, in most cases), email me with them and I'll host them here, and then email you the links back, so you can insert them into a post.
If your pics are larger than about 1MiB, upload them via my "large files" page:
http://asavage.dyndns.org/work/uploadForm1.html
along with either an email or text note, telling me what they are and who you are. This is a manual system, not automated, so it's a bit of work.