The picture needs to already be on the www somewhere. To do this, people use sites such as Photobucket - these are simply free online photo albums. You need to create an account, then upload a photo.
When you upload a photo and look at it, you'll see its 'URL' - it's web address. It's at the top of the picture where web addresses usually appear, or sometimes the site also puts it in a little box. You need to highlight ALL of the URL and copy it.
Then when you come to AB you just paste.
The link appears and people who click on it can go to the picture.
If you paste a Youtube link inot the 'enter Youtube' under your question, the whole video appears. This is deep old magick arranged by the Big Ed.