The photo has to be hosted somewhere (an image server) which is always connected to the net - Flickr, Photobucket, or another image host. These are usually free. Go and sign up to one, and full instruction on how to upload your photos to them will be there.
Once you've uploaded your photos, you'll see an album with the photos and different ways of linking to each one. I haven't used Flickr, but with Photobucket there are four different links - the easiest one to use is 'direct link'
When you click 'reply' here, you'll see a couple of rows of control buttons on top of the edit box. Click on the one that looks like mountains with a sun in a yellow sky, and you'll get a little box up that you can paste your direct link into. Do that and click OK, and that's your photo posted.
It sounds a faff written out here, but it's easy in practice - the most hassly bit is setting up the image host account, and you only have to do that once.
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