In general, try to use images that match the resolution and aspect ratio of the largest screens in your environment. In most workplace environments this will be landscape-oriented TVs and monitors set to 1920 pixels wide x 1080 pixels high resolution, a.k.a. "1080p". Thus, if you upload 1920x1080 landscape-oriented images you'll get the best-looking results on those big screens.
You have the option to devote the entire screen to your images (a.k.a. "fullscreen") by hiding the Title Bar and Ticker.
Higher-resolution images are also OK, as the image will be scaled downwards to fit the screen. You can control image scaling explicitly using the Image Layout options.
Lower-resolution images may look fine too, although if you go too low the image may look grainy or too small to be comfortably visible on a large screen.
DIVE can accommodate landscape and portrait aspect ratios, and a variety of screen resolutions up to and including 1920x1080. DIVE currently looks best on a landscape-oriented 1920x1080 screen.