Oh, I see. You have to pay to host pictures on livejournal, but if your photos are on flickr or your personal website or anywhere else, you can link from Livejournal without paying. There should be a button marked 'insert image' at the top of your text entry box, and you just cut and paste the url.

Or you type lessthan img src="url" morethan (but using the signs instead of the words) and just fill your picture url in.