[ubuntu] Wallch wallpaper changer

Have installed Wallch on a newly installed 16.04 x64, but can't get it to show any pictures.
Seems to run ok, and allows me to configure it, but I never get a wallpaper (have the desktop configured otherwise as just a solid colour). Even in the setup page, I can see the thumbnails of the pictures (just standard photos, straight off a number of different cameras), but clicking on a thumbnaiil, in the preview window, just remains blank, and clicking on a pic and selecting set as wallpaper does nothing. Apart from that, no errors etc.
Thanks.