Custom fields are available for Images as well as other modules. There's a link at the bottom of the sidebar when you go to Content -> Images in the administration menu.
If you add a custom field of type "Long text", shortening the description shouldn't be a problem. You can also tweak the database type which will be used for that field in the Advanced options when creating it.
For the URL, even a "Short text" field should suffice.
Here's how to find the link to edit custom fields for images: http://screencast.com/t/grxXesYurW
All the best,
the Telerik team
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