In order to link to a page for an image you need to use a File Attribute. Go to Dashboard > Files > Attributes. You can reuse the text attributes found in the list to however, to make our attribute descriptive let us create a new Text attribute and then hit Go.
After which you need to specify the Handle and Name of the attribute you can follow the information below and then hit Add.
Once you are done you should be redirected to the previous screen and then you should be able to see the attribute you created in the list.
You can now go to Dashboard > Files > File Manager and then search for the file that you wanted to link a page to. Once found click and choose Properties in the context menu.
Once clicked you should be able to see the Properties window launch. And in that window you should find the attribute you have created and click on it and you should be able to edit the value and specify the URL you want. Finally hit the the save button to commit changes.
Going back to the block interface you should be able to see the attribute in the Custom Attribute dropdown and specify which attribute to get the URL from.
Hit save then reload the page and that should do it.