Hacked – Splash Screen in dashboard

Looks like my site has been hacked. The site appears fine in public view, but the dashboard is covered with an image. The image is hosted on someone’s dropbox account: https://uca130762f54e84a868270ea4a90.dl.dropboxusercontent.com/cd/0/inline/AaboDoM3_IXGTh5hIzjYRI9bhcgth62WFmqJ02sLUt1v-UNflQ-ekNIEcMVGBKpc1JE8dF64aHam9PUR4Vc59sHyRG6UJ7e3OZ_yuG7AMOEBYQ/file#

Here’s what it looks like in the dashboard.
https://drive.google.com/file/d/1MeGYasfh9573Mk0mhoFqwkv77_UqAQ-k/view?usp=sharing

What is this and how do I get rid of it? I have changed all passwords – wordpress and ftp, removed all plugins (to eliminate any malicious plugins), admin has been disabled for years. Anything else?