ZDoom

So how do i change the main menu background?
Fo i find the exact texture of it and override it via my wad?

Is there a way to have this texture randomised with few textures?

Open your wad in Slade, import the picture you want to use in and name it TITLEPIC. Kappes explained this better in an old post.

Never seen any wad/pk3 with more than one image as background for the main menu, though.

Yeah thanks, i found it right after i posted thos.

What abot changing background images for every level i create?

If you are refering about how to change the sky (called skybox) in each level, that's something you set in MAPINFO (if you don't have said file in your wad, create it via Slade).

This is an example taken from the wiki. First you import into your wad the picture you intend to use as replacement for the sky and then write the exact filename in the line that starts with sky1 and then repeat the process for each map specifying that same line with the name of each different picture you prepare:
It may seem difficult to understand at first (here is a description of what does each line means) but you don't need to specify each and every one of the properties of the map like in the example, only the ones you need. You can always open maps from other people and see what they did to use as reference.

On the other hand, if you meant something like having an image appearing before/after each level, then you are looking for "cluster" instead of "map". Both are set in MAPINFO, anyways:

Ivwas referring to more like:

In doom 1, on every level i exit and enter, it showed this "map" of my journey in game through every levels. From develompent stsnd point, i think that map was only a form of a picture. So therefore, with every level i complete - different image file i guess

That is the intermission screen, also configured via MAPINFO. Depending on how you expect the intermission to be, honestly. Do you want a custom "map" like in original Doom that changes after each level to show your progress? Are you going for a simple picture with some text narrating the story, perhaps? If you've seen a mod with just the same idea you want to pull off, you can open said mod in Slade and check its MAPINFO lump to see what did the author do.

Or share it here in the likes of "I want to do something like in the mod yaddayadda with this cool intermission".

More precisely, it's the [wiki]intermission script[/wiki].