This isn't a bug, but just how the art library currently works. When you upload an image to Roll20, we store multiple different versions of that image - the original version image, a medium version that has either a max dimension of 512 in a given direction, and a thumbnail size. So your browser isn't constantly processing full sized images constantly, the tabletop swaps in the needed size depending on how much space the image currently takes up on the browser. When you upload content from your local files, it drops down the original version of the image. If you drag content out of the art library, it defaults to a single grid square (if the grid is turned on). If you're holding down the Alt key while dragging it will place the Medium sized version of the image. You aren't the first user who has made mention of this inconvenience, so giving GMs the ability to drag content as their original file size (probably with a certain keyboard shortcut) is something we will likely try to do in the future when we do another pass of Tabletop updates.
