Through trial and error, I have found that the GetPixel function takes two arguments, one for X and one for Y, even if used on a 1D image. On a 1D image, the second index must be set to zero.
image list := [3]: {1,2,3}
list.GetPixel(0,0) // Gets 1
GetPixel(list, 0, 0) // Equivalent
How am I supposed to know this? I can't see anything clearly specifying this in the documentation.
This is best done by using the script function with an incorrect parameter list, running the script, and observing the error output: