I noticed on the free version of the catch kathmandu wordpress theme. There is was no option for a full background image. This is easy to fix. All you need to do upload the background image use, in the “Custom Background Color” section. This can be found here: Appearance > Background. Then just make sure to put select no repeat. Once you do that go to the custom css section and paste in the following code:
background-size: 100% 100%;
Then you should have a full width background like this one.