To use an image as a tilesheet, simply drag the image from the assets panel, onto the tilesheet panel. The map will now use this image as tilesheet. Note that a tilesheet needs to be arranged in a 16 by 16 grid of tile. Looking at your screencap, you will need to edit the image to remove the gaps between tiles, and rearrange the tiles in a 16 by 16 grid. Also, if the tile size is not 8 by 8 pixel, you can change that in the project settings.