Hello everyone first time posting, as I've come up short at a task larger than what I've done so far on Mathematica.

Here's my situation:

I have taken a picture of a laser with a detector I would like to use mathematica to find the best-fit function for intensity of the beam (brightness of pixel) with respect to the Cartesian coordinates of a point on the image, so that I can compare the results with the theoretical Gaussian distribution of intensity for the laser.

I was thinking that using ImageData to conver the pixels to an array might make sense to convert the picture to datapoints for Mathematica.
Does anyone know how I might find the multivariable best fit for data in the format?

It's also worth noting that the image is 3600x2700, so trial solutions will take my computer quite some time.

EDIT: I am encountering a problem when I go from RGB data to Intensity. There seems to be an extra data point on each pixel. What I want is to convert the image to data in the form of a matrix where each element is the corresponding pixel's normalized intensity. I can do this with pictures from the web, but when I do the same process to my data, I get a matrix of lists of two elements. How can I eliminate an element of each element of a matrix?