How could I change background with this Tumblr background?

It is using a external CSS file that you will not be able to edit. But the good news is if you add it it will work.

<style type=“text/css”>
.regular img {
max-width: 100%;
}

</style>

Just add it in there and it should override what is in the external file. If it doesn’t add !important to the background property you add.

Where exactly should I add this? It won’t work…

Customize -> Theme -> Use Custom HTML

http://i.imgur.com/OCfD2.png