The elements of MATLAB figures (AKA handle graphics) or of diffent types. E.g. a figure with two subplots typically has two axes. All thos elements have handles to identify them. In your code you need the "number" of the axes - the handle.
If you have just one axes, you can use the GCA command.
Note that the syntax of MATLAB's built-in function is:
image(x,y,C,'PropertyName',PropertyValue,...)
