Please put the backgrounds back. Two screenshots is nowhere close to enough information for us to begin to figure out what is going on. However, my very broad guess is it is a combination of what you are assigning the background to and the value of "background-size". Based on the CSS your posted, it looks like you are assigning the background to the entire body which is not necessarily limited to the visible part of the page. As for "background-size" the option you have selected fills the entire area even if it means distorting the image; so the distortion is no surprise.
I am still not seeing any of the backgrounds as-is but I think I have identified why. The issue for both the main page and the Marvel Comics pages is that your value for "background-image" is being overridden by a rule with higher specificity. As for the distortion, it is indeed because you are using "background-size: cover;". One more thing, in general, it is a good idea to stay away from using "!important". Using it to get things working seems to be a common bad habit among FANDOM wiki editors. However, the better solution is to increase the specifcity of your selector. Try this instead of your current CSS: