I would like to create a nice portfolio item like below. But I'm a developer and no experience with graphic design. But I would like to create portfolio items. i.e. no need to be a professional designer. I just need to learn something to do this task. Could you tell me what kind of tool should I use for it? Do I have any free tools to create such portfolio items very easily?
tl;dr: I will have a web site and I would like to create its front screen as portfolio item on mobile, tab, and desktop as below.