You then need to close the HTML tag with this: < /a >
Then check it in the preview window. It's always a good idea to add a line or so of text after the HTML stuff, to ensure that you've closed all the tags. Then after checking in the preview window you can delete the added text if it's not needed.
You have it right the first time. That's all you need to post a picture.
JUST < img src=http://URL/filename.jpg > (with the beginning and end spaces closed.)