The current build should work as inteded.
First you need to select your /workshop/ folder in Steam/steamapps/common/wallpaper_engine/.
Second the program should load the files on the right panel, now you should see names, pick one that you wanna remove and press Remove on the left panel.
By the way, you should be able to see the Wallpaper preview when you select one of them.
If you see the mosiac (pixels with random color) then that means the preview images wasn't avaliable or it doesn't exists.
Currently theres only Windows 32-bit build.