Fancy Image links is a really useful, cool and (did I say "useful") add on for concrete5. Make an image a link to a page on your site, another site, a document or nothing at all. Admittedly that last option is not really an astounding feat but... you never know.
You can set the max width and height (the greater will take precedance) and youcan have it open in a lightbow with a dimmed window behind it.
The price is free so you have no reason not to get this gem.