There are two ways to post a photo.
One is to have it uploaded somewhere (Flickr, Photobucket, etc) where you can get the URL for the image itself. (Easiest is to have the picture open, right-click and select "copy image url" or right-click and select "properties" and highlight the url then right click and select "copy".)
Once you have the picture's adderess, you can click on the "insert image" icon in the reply window:
... and it will give you a pop-up to paste the picture's address.
The other method is to scroll down the on the reply page -- under "Additional options" there is "Attach files". Click on the "Manage Attachments" button and then you can either pick a file from your computer or you can paste in the address of the photo online. Either method uploads the photo to knittinghelp and it will show up at the bottom of your post as an Attached Thumbnail.