What is the recommended image dimensions?

Creat support service :D

Nothing about dimensions when you upload nor on the "information" page:

https://support.kahoot.com/hc/en-us/articles/115002815387-How-to-add-and-edit-images

I also would like to know that. I would like to make the slides background in e.g. Illustrator. What's the optimal resolution?

Go for 1530 x 1020. Or if you wish to have a higher resolution , to get the correct aspect ratio:  Width / 1,5 = height