I deleted a picture, but Variety downloads it again. Why?

Variety bans the URL the picture came from and never downloads from the same URL again. But this same picture can be downloaded from another source, and also some pictures are duplicated on Wallbase and the other sites (often with slight modifications in size, brightness, etc.), and are thus available at different URLs. Variety does not have complex logic for recognizing variants of the same image.

You can see the list of banned URLs in ~/.config/variety/banned.txt. Watch it increase as you delete more images to Trash.