How to change User picture in Preferences?

Users & Groups User preferences on Mac - Apple Support says: "Instead of clicking the picture, you can drag and drop an image file from the Finder onto the picture." But when I do that, the picture just bounces off. I think I've done this before, and it worked (I know I've placed this particular image in the User picture on other occasions). But not this time. I've tried both .png and .jpg images, neither works. I've also clicked the lock and entered my admin password, though I'm not sure I had to do that before.

I guess I could create a photo database in Photos, but I don't want to do that. I just want to do what the instruction above says: drag a little picture from the Finder onto the User picture to replace it. This is a fresh install of 10.13 High Sierra (on a 2013 MacBook Pro, otherwise working okay), complete with updates; I don't want to start cluttering it up with other stuff just yet.

Anybody have any ideas? TIA.