option 1 - attach a file or files to your post
on the page where you're entering the text of your post, you'll also find an "Upload Attachment" section. in the filename field, you can click to find photos on your computer, and they will be attached to/displayed in your post.
option 2 - link to a photo which is hosted externally
if you already have an online photo album at flickr, smugmug, etc., you can find the url for your photo on that external site, and you can include that url inside an "Img" tag: first, paste the url into your post, highlight it, and then click the "Img" button above (in the formatting bar).
hope this helps. if it's incomplete or confusing, let me know, and I'll try again...