In OBS, if a Source (such as your dog images) doesn't fill the screen area (what OBS calls the "canvas"), then OBS shows whatever is "below" or "behind" the Source. If your Scene has multiple Sources, it will be the next Source below. If there are no Sources below, then you see black.
You have a couple options:
1) after you crop to remove the stuff you don't want, resize and recenter the cropped image to fill the canvas
2) if the shape of your cropped source doesn't match the shape of the canvas, and you don't like black borders, put another Source behind your image. The "Color Source" lets you pick any solid color, for example. Note that when you add a Source to a scene, OBS puts it on top of any existing sources in the scene. You can use the V-shaped and ^-shaped buttons at the bottom of the Sources list to change the order.