If you would like the files to be displayed with a proper artwork on your iOS device, try this:
Create a folder with the .png image having the same name as the file itself and make sure to drop the file and the .png to the folder.
For example, you want to transfer a song called 'My Way' by Frank Sinatra. Let's say, the .mp3 file is called Frank_Sinatra_My_Way.mp3. So, the cover art file should be named Frank_Sinatra_My_Way.png.
After you named both files in the proper way and pushed them to your folder, drop the folder to WALTR2 Mac.
Should be all good now!