Let's populate the board by uploading some assets to Bonfire, open up the Asset panel by pressing A or through the toolbelt, and drag-and-drop files from your computer directly on it. You can drop multiple files at the same time.
You can also drag-and-drop files directly onto the Board to both upload them to your Bonfire account, and place a sprite on the Board.
As of the current alpha, allowed files are jpg and png up to 3MB, you can upload a total of 100MB of files to your account.
Drag-and-drop assets from the panel directly onto the board to place them. Hold Shift while dropping to resize it to the grid's cell size, hold Control to make it hidden by default. You can combine these shortcuts.
Assets can be renamed and deleted through the Assets page in your account.
Deleting an asset will automatically remove it from all boards where it has been used.