Is there a way to get gallery images to link to a post or custom page?

Hi guys, trying to set up my portfolio theme (Gridster-Lite) at my site here and I have a question about galleries. When I insert a gallery onto my portfolio page, I only have the options of linking the gallery thumbnails to either a larger version of the media file, or an attachment page with the media file and limiting editing capabilities.

What I’m trying to figure out is if there’s a way I can instead link the gallery images to specific posts or pages within my site. For instance, I design book covers, and I set the gallery thumbnail dimensions to best display the front of the covers….but I would like clicking on those thumbnails to take people to a page where the full wraparound cover is displayed. However, I can’t find a way to switch the image file on the attachment page or to redirect the link to a page where the full cover is viewable.

Does this make sense? Anyone know of a fix or a workaround for this?