I searched around but didn't find anything definitive on this. (This info is for a 2018 model, 12.3" screen)

I was playing around trying to find the right size for the start up image. I did find a good size so if you use this aspect, you should be fine.

1200 x 675 pixels

The problem is that it doesn't fill the screen. The image only fills the center part. I try about 30 sizes and it limits it to this region.
So if you want to upload your own images and have them look like they fill the screen, they need to have a black area around it. It won't fill the screen but won't be so noticeable.

The pics below show the difference between 1200x670, 675, and 680.
1200x670 leaves a very slight gap to the top and bottom of the placement border.
1200x680 leaves a very slight gap at the right and left of the placement border.
1200x675 appears to completely fill the placement border. (if it doesn't it is off by 1 or 2 pixels and I can't see it)

the other 2 show how much of the screen is filled. I had to use a flash as it was just dark enough to not be able to really see it.

 

Great post on the different background startup image sizes! Thanks for sharing your findings. This will be very helpful who decide to do this in the future.

~ Im2bz2p345