Your pictures need to be hosted on a photo hosting website. An easy one to use is Tinypic
Upload your picture, and choose the resize for message boards size. Then click upload now.

After you upload the picture copy the HTML for websites code and paste it into your post.

Hi INMC,
Would love to see your glass tub shower enclosure as I am stressing about what to do with my guest bath remodel since I can't hang shower curtain rod.
If you have a google account, you can also pretty much follow terriks instructions to post photos and then embed the link. I mention it only because it's also free and I use it for google checkout too so it's a multi purpose account.

There's a test forum also - where you can test posting photos until you get it figured out. That's what I did when I first learned to do it. When you're back on the bathroom discussions page if you scroll to the bottom you'll find it. I use both photobucket and webshots for my photos. They've made it a lot easier now than it used to be.