Why did my desktop background changed and why pictures were opened, if I did nothing to do so?
Do you have the same image selected for all Spaces? that would be the simplest explanation, that you don't.

Test by pressing F4 [or FnF4 depending on how your machine prefs are set]. This should zoom all the windows out so you can see them all, but also present a set of you current Spaces at the top of the screen.

If you mouse over those, they will expand to small pictures &amp; unless an app is covering the entire Space, you will be able to see the desktop background for that Space too…

For example…



All my Spaces are set to the same image, but it's not compulsory.