You can use your own image for albums with no artwork.
See this guide about how to override skin files: https://getmusicbee.com/forum/index.php?topic=22023.0
What you need in the third line is
<element id="NoArtwork">xxx</element>
And replace xxx with base64 string of the image.
Replacing the popup window is not possible, nor is possible to edit the buttons. Request it on the wishlist board.