Hello Emily,
MATLAB looks for files on the MATLAB Search Path. If a file is not on there, or in the current directory, then you'll need to provide an appropriate relative or full file path to the file you would like to operate on.
For a Mac, you can probably use something like:
I = imread('~/Desktop/pothole.png');
When dealing with code files, it is pretty standard to put them somewhere on the search path, if they are to be used as utility functions by lots of other code you write. For data files, it is more common to provide the full file path to the file.
-Cam
