I'm not a mac user, but I assume the procedure is the same...
Choose a photo hosting site. I use Photobucket, but there are others.
Register, and follow their instructions to upload the pic (on photobucket, I browse for the file name, choose it, then hit the UPLOAD button on screen. Very simple)
Once it is uploaded, you will see your picture, and under it will be several different sorts of links. Choose the one that starts with [ img ] and ends with [/ img ] (without the spaces), copy it (in photobucket, this happens automatically when you click on the link) and paste it into your post. The picture will appear at that point.
The other common option is the web link, where you copy the web address of the pic (which starts 'http:....') and paste that in, and people click on it. Better for images that people might not want popping up on screen, say because they are not worksafe, or require a warning....
Is that any clearer?