Background GIF image converts into jpg

Why is that? It says GIF format is allowed and then for whatever reason it changes it into jpg.
And it is smaller than 2 MBs.

Where do you add the background and how?
Do you use admin panel or CSS?

Also post the background image here.

I uploaded the image to ACP - Display - Pics management - general/explore - Background picture (page) - Upload a picture. And I pasted the code there.

I have always done it this way with the background picture cause if I upload it somehwere else it resizes the pic.
The image that converted was 681 KBs, 1599X571.

I've managed to get in gif and in actual size now but I had to upload it to tinypic.