Yes, it is possible to replace all graphics.
You need to create an image, and add some ruleset (text description).
There are many mods around you can take inspiration from.
I'm attaching one sample mod, which changes the background for research GUI (bgtest.zip).
Please note that most graphics in xcom are 256 colors or even less, for example the backgrounds you mentioned are only 16 colors (and you need to use very specific palette indexes for this to work... look at the palette of the sample image (Back_05.gif) to see what I mean).
PS: you can't use jpg... only gif and png... I recommend png