For pixel art, you must NEVER save as JPEG. In addition to that, your builgin is 'flat'. There should be differencies in brightness on the various sides, and you shouldn't copy elements. Even though 2 windows have the same size and shape, reflections, and what you may see through it, as well as light angle, will differ.
As far as game maker goes, it needs some effort to get through it's basic workflow, though within a week or so you can start making your own game IF you have the ability to think out the systems required. I don't know what you're planning, but for example good pathfinding doesn't exist in gamemaker, nor any other software I know of. You'll always have to think it out and program it yourself, all gamemaker does for you, is ease the latter. Not the first. So you need general ability of logical thinking, or you end up asking everything on forums. But even if you do so, it may turn out an ok game, so then all that matters, and also all that is relevant on these boards, is the art you will create. Not the game. Though just to awnser your first and second questions: yes and yes. That's the simple part.
Welcome to pixelation, and good luck.