If you don't find the exact layout you’re looking for with the templates available in the Canvas, you can alter the existing layouts to suit your needs. The current layouts are designed as a starting point, you can, for example, add an extra column, merge two cells and make other layout changes as needed.
The right click menu is context sensitive, if your mouse is over an image, the options will relate to image adjustment, if over general area of the template you will be able to add to, subtract from or merge aspects of the current table.
To combine two cells drag your mouse over two adjacent cells and you should see a light blue selector around both. Right click with your mouse and choose Merge Cells to make one large cell.