If you've got a photo you want to use already residing on your hard drive, you are more than half-way home!
To post a picture in a post, just set your cursor where you want it, click on the "picture" icon (farthest left icon in the bottom row of the Reply Box), you'll see a pop-up box to "add/upload an image". Click on the browse button, find the pic on your HDD and click OK.
If you want to set an avatar image do the same basic thing but click on the "My Profile" tab at the top of any HTF page, click on "Edit Community Profile" and click under your image on the "Change Image" link.