The Billboards block allows you to link all of your images to just one files detail page, or you can link each image to a seperate URL. Here's a quick tutorial on how to do the latter:
1) Go to your dashboard ( /dashboard/files/attributes/ ) and add a new file attribute that'll contain the custom link url for an image. In this example will create one with the handle image_link_url and a name of "Image Link URL". The file attribute type can be either "Text" holding a full url, or can be of type "Number" holding a page's collection id.
2) Open up your images in the file manager, open the image properties, and set a value for its image_link_url attribute.
3) Edit your Billboards block, scroll down to the "Image Link URL Attribute", and select "Image Link URL" from the select menu.
4) Save your changes and it should be good to go