There should be a button called "Add Image Overlay." Click that. A menu will pop up. Click the "Browse..." button and select the map you want to use. You'll see the map but it won't take up the whole globe. Use the green lines on the edges to resize it to take up the whole globe. Be careful when dragging it up to the poles, it's very finicky about that. Then, instead of making a new overlay to make another globe, just copy the current one so you only have to set it up once. If you ever want to edit a current one, just right click it on the left and select "Properties."
Hope that helps!