I know how to create a DEB and push it into my PPA at Launchpad but I want to know how I can include a screenshot of the application that it shows up in Software-Center.
If you want your application to have a screenshot in the software-center, it has to be in the repositories.
If you are interested in getting your package into the repositories, follow this question.
Once you have the package into the repository, you can follow this question to add the screenshot for your package.
The Software Center pulls screenshots out based on package name from a central screenshot server, so it's the same as any other package, you upload images through the Ubuntu Screenshots site:
However that site will only let you enter in package names that already exist in the main repos. If you're developing a new package, you're going to be screenshot-less until you're in the main repos.