Links to images may be used in either the target or the description, or both. Thus, the following code will publish as a clickable image that points to http://www.mwolson.org/.
[[http://www.mwolson.org/][http://www.mwolson.org/static/logos/site-logo.png]]
If a link to a locally-available image is encountered in the link description, Muse mode will attempt to display it if your version of Emacs permits this. The following example will display correctly and publish correctly if a PNG file called TestLogo.png exists in the ../pics/ directory.
[[TestPage][../pics/TestLogo.png]]
An image link is not required to have a description. The link [[../myimage.png]] will display and publish as expected.