The best way is to specify a URL from the available options while you are inserting the image.
Otherwise, to turn an existing image into a link, create a text link (any text), then reposition a photo inside the linked text, then delete the link text from around the photo. Be careful not to delete from the end of the link, or this will delete the link. Using the delete button is often better than the backspace button.
In most web browsers, you can also highlight an image from moving the mouse from left to right over the image, so that it is grayed out or ghosted. Then you can link it in the normal way. (Note If you click on the image, then the borders will become active and you can resize the image, but not link it... so you need to do the mouse drag thing)