Just some quick suggestions for getting your upload screenies looking good:
Frame the shot: Make sure your subject is nicely framed. This doesn't necessarily mean everything is dead center. Sometimes putting a subject off to the side a bit or in a corner is nice way of revealing a bit of the environment around it.
Give it some drama: Capture characters mid-pose. Position objects angled toward the camera rather than simply head-on or sideways. Low angles tend to make things look bigger and more dramatic. Don't be afraid to get in close. Maybe you're taking a shot of a car... don't be afraid to crop a bit off and start on the hood rather than show the whole thing.
Take advantage of the aspect ratio. The aspect ratio of the displayed images is rather wide at 17:9, so get in close to crop out excess environment and add drama. Rather than show a full character, maybe zoom in on the eyes...
speaking of the aspect ratio: displayed images are 340x180. Simple math, but here are other resolutions you may crop to before resizing for your best image. 510x270, 680x360, 850x450, 1020x540, 1190x630, 1360x720, 1530x810.
Looking at 1020x540 and comparing to standard res of 1024x768, you can see you'll need to knock off more than 200 pixels top and bottom while leaving the width intact, so keep that in mind for your initial framing.