Hi I am trying to add a url to an image tag with a wordpress shortcode that I already have... How can I get it to work?
You cannot use Shortcodes in attributes since 4.2.3, for security reasons. See the announcement (emphasis mine):
Earlier today, we released WordPress 4.2.3, which includes a relatively large security fix that affects the Shortcode API. Due to the nature of the fix – as is often the case with security fixes – we were unable to alert plugin authors ahead of time, however we did make efforts to scan the plugin directory for plugins that may have been affected.
With this change, every effort has been made to preserve all of the core features of the Shortcode API. That said, there are some new limitations that affect some rare uses of shortcodes.
In today’s release of WordPress 4.2.3, however, we’ve added some new limitations that affect some existing plugins. Take, for example, the following shortcode, which is no longer recognized:
<div style="background-image: url('[shortcode]');">
The shortcode in the example above appears in a context that is no longer supported. Further, this use of a shortcode stretches the imagination for how the Shortcode API was intended to be used. Fortunately, there are some workarounds still available, so that site administrators are not overly restricted in their use of HTML.
Read the full announcement for the workarounds.