In the image picker and on Your images, a click on a thumbnail that moved even slightly, as it easily does on a trackpad, could drag the image and drop it back on the page. That uploaded a smaller copy of an image already in your library. Thumbnails can no longer be dragged, and only images dragged in from your computer or another site are uploaded.
Copies made before this change are still in Your images. Their names end in -1, -2 and so on; select them there and delete them.