I have not installed base launcher in a long time so I might miss a few files. When you first open the launcher on a machine that has not had the launcher on it before you get an option to where you wish to install the packs. The recommended place is a folder you create named FTB on your Desktop. What you decide then is where the mod pack folders are put. Inside those folders, like you said, is where your screenshots and saves are. Now the launcher keeps information about these options inside the %AppData% folder in its own folder name ftblauncher. Inside that folder is where the launcher keeps its information about where it is to look for mod packs and what you've selected as far as download location, arguments, language and other options.
I hope that is what you wanted to know and was explained well enough. I'm a little groggy.