Custom login page background image

I would like to customize my login page with my own background image with the Theming App.

What are the recommended properties for a custom background image (pixels, aspect ratio, file format)?

I try to keep the file as small (in size) as possible and the resolution as high as users’ monitors…

I’d like to bring this subject up as I have found that using a small file does not have any effect.
Let me explain. I’m using a custom image for login page. I used a photo that I took which originally weight around 8Mo. I compressed it to less than 1Mo and uploaded it in the parameters.
However when loading the login page, it’s take a long time to show the background so I checked it and it actually loads a 11Mo file ! I tried different format, size, and compression, and it’s always the same. Moreover, the max-age of the picture in cache-control is only an hour, which means it basically has to be downloaded each time the user needs to login. This is really infuriating and I have not found any workaround.
Do you experience the same issue ?
I shall open an issue about this because I can’t see a good reason for this behaviour and it may need further investigation.