Locally, just create the ScreenshotsFolder in the same directory level as AppsFolder and README.md.
Then, git add ScreenshotsFolder
will add the folder and all of its content (really, all content that is not in your .gitignore file if you have one).
Next just git commit
then git push remote_repo remote_branch
(ex: git push origin master
).
If you aren't doing this via the command line, I'm sure the EGit plugin has some option to add the ScreenshotsFolder and do a commit then push...haven't used EGit myself, though. The steps are the same either way, just a different interface.