Background to my web/app - tool


It’s not a typical UX/UI question, but maybe someone would be able to help me.
I want to work on some UI hi-fi prototypes in my free time and I am wondering what tools I can use to create / paint a background to my landing page / app. I want to create something like this:

If you know any software that I can use, I would be grateful for any tips! Cheers!