Peak analysis of 3D images

I am trying to analyse an SEM image which has x, y and intensity information. I am looking to locate peaks in the image where I have observed particles but I cannot seem to do this in MATLAB.

I have tried '[pks] = findpeaks()' but it seems to me as if this is designed for analysis of an xy matrix not an image?

When I run '[pks] = findpeaks(I3)' on my image I get the following error messages:

Error using findpeaks Expected X to be a vector.

Error in findpeaks>parse_inputs (line 70) validateattributes(X,{'numeric'},{'nonempty','real','vector'},...

Error in findpeaks (line 59) [X,Ph,Pd,Th,Np,Str,infIdx] = parse_inputs(X,varargin{:});

Any ideas how to fix this?

Thanks Rob