Image Button Add-on v2.0.0 for WPDM Pro v4.0.0 has been released, here you upgrade the image button add-on totally, Now you will be able to set separate image for download buttons in link and page template/details page. If you are already using WordPress Download Manager Pro v.4.0.0 RC1, please download image button add-on again to get the latest version of the add-on and install.
Image Button Add-on Screenshot: