I know what this is.
You’re using my transparent theme, and since it for unknown reasons can’t be installed by some people I posted the CSS preloaded with my settings, which has that image as the default avatar. You can set it back to default (and I’ll edit that post with the CSS as well) by putting this url https://discourse-cdn-sjc1.com/business5/uploads/wanikanicommunity/original/3X/f/d/fd4c154120954695f788402f3bcf4e616499bc2d.png here in the style
