There are only a few water heaters which do not use standard elements which can be purchased at your local box store or hardware store. Most elements are the screw in type though some are held in with four studs with nuts. Removing one of the cover plate over the thermostat/element will tell you all you need to know.
A screw in type takes a 1.5 inch socket or you can buy the six sided tubing wrench at the store which has the two holes for inserting a large screw driver.
Picture three shows me what I think is a piece of 10/2 which means you can have either 4500 or 5500 watt elements. Again, the wattage should be listed on the side of the element. Sometimes there will be a dual rating: 4500/240; 3380/208.