There are two ways:
1. upload the photo to an image hosting site, I use imgbb.com, copy the HTML link provided. Click the 2 blue arrow button on the bulletin/blog editor to open the HTML editor for the post, and paste the code there.
2. upload the photo to an image hosting site, get the plain link with the filetype at the end (NOT "viewer link") provided in the html code or BBcode (you'll have to get rid of some parts just to have the link, some sites may give you the link alone, depends). click the photograph icon in the editor and paste the link there.