Hello, I wanted to change the background in Deepin 20.4 Boot menu! Drag and drop an image! But when you log out, you can't see anything! Who can help? thank you

Drag and drop where?

general settings-boot menu-background!

...and...
Your question isn't very clear. When you log out, you will obviously not see anything, because you are LOGGING OUT. The boot (grub) menu image comes up when the system is BOOTING, not when you log in. The login splash screen is different than the grub image. The Deepin docs tell you how to do this:
https://wiki.deepin.org/wiki/Deepin_Desktop_Environment

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Boot Menu Theme
On the homepage of Control Center, click .
Click Boot Menu.
Switch on Theme to use it as the theme of boot menu.
Tips: You can drag and drop an image to the preview window to change the background of boot menu.

Window

If you want to change the background image on your machine, then follow the directions that you were given in the documentation that was looked up for you. Again, this is an English forum...posting a link to a website that's in German isn't going to get much.

Beyond that, you need to read the "Question Guidelines" link in my posting signature. We are happy to try to help you, but you need to do basic research on your own first. There are ample instructions in the link you were sent that cover changing login/logout screens, grub boot menus, etc....all from the Deepin site.

Maybe it doesn't support drag'n'drop. Have you tried another way, like clicking a button that says "Choose File" or some such?

(Sorry I'm not clicking that weird blog link you posted)

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Hello, the log screen doesn't work either! change of Bachkround not possible!

Yes it is possible. Again, you need to read the instructions from Deepin that you were provided earlier, and follow them.