When you click the image button in Discover, there should be two tabs: "New Images" and "Uploaded Images." When you go to "New Images," there should be two ways to upload an image.
If you click in the "Click to upload" rectangle, your browser should open a window to let you navigate to the image file.
Alternatively, you can upload images with drag-and-drop. Just drag the image from its location into the "Click to upload" rectangle and drop it in there.
Does that help?