If your matlab image is 'RGB' matrix, you can use following matlab function block + Bus assignment block as shown in figure.
function [H, W, Enc, EncLength, IsBig, Step, Data, DataLength] = fcn(im)
[H, W, n] = size(im);
H =uint32(H); W =uint32(W);
im = permute(im,[2 1 3]);
Data1 = im(:);
Data2 = reshape(Data1,[H*W,n])';
Data = Data2(:);
Enc = zeros(1,128, 'uint8');
Enc(1:4) = uint8('rgb8'); % uint8('rgb8')
EncLength = uint32(4);
IsBig = uint8(0);
Step = uint32(W*n);
DataLength = uint32(H*W*n);
See the attached simulink readymade block also to convert MAT image to ros image message.
