Change signin screen to a certain image keeping lock screen slideshow?

I have the option for 'Show lock screen background picture on the sign-in screen' enabled, but every time I get to the sign-in screen the only image that is ever displayed is the default 'beach from inside a cave' picture. My current settings are that it will choose images as a slideshow from a specific folder which definitely does not have the beach picture in it, and it works just fine for the lock screen itself.

EDIT: I changed the lock-screen settings to only use a picture instead and it worked just fine, but now the lock screen only shows that image, not a slideshow, regardless of settings. Is having the sign-in screen as a slideshow even possible?

If I do that then the normal slideshow works and the sign-in screen just has flat colour. I would really prefer if there was a way to get a certain picture to show up there that isn't the default one while keeping a slideshow on the lock screen, but I guess that's just not possible in this version of Windows for some reason.