You are correct that readImage and writeImage functions are for MATLAB use only since they work on ROS topic objects. However in Simulink we work with buses. The straight forward way that I could think of to deal with this is to unpack the image on your own in a MATLAB function block. So you could have a model like this:
And inside the MATLAB function block, you could write the following function:
function img = fcn(msg)
%#codegen
%Get msg details
imgData = msg.Data;
imgH = msg.Height;
imgW = msg.Width;
%Create blank image output with required size
img = zeros(msg.Width,msg.Height,3);
%Reshape image data
imgR = reshape(imgData(1:3:end),imgW,imgH)';
imgG = reshape(imgData(2:3:end),imgW,imgH)';
imgB = reshape(imgData(3:3:end),imgW,imgH)';
img(:,:,1) = imgR;
img(:,:,2) = imgG;
img(:,:,3) = imgB;
Note that this example assumes image is in the RGB color space.
