To make a clickable picture you just need to nest your tags..
PHP Code:
[url=<the link you want>][img]<image source>[/img][/url]
For example
PHP Code:
[url=http://www.mobileread.com][img]https://www2.mobileread.com/i/mr/misc/logo.png[/img][/url]
Gives:
NOTE: Its not PHP code, but that was the only way I could get it to display the source code right.... not sure why the "CODE" tag did not work.