Error using listbox multiselection for image SNR manipulation

I am trying to use a listbox multiselection to add images together and median combine for SNR improvement comparison reasons. I first allow the multiselection from the list box using:

%Get selected files from listbox selection
 set(handles.listbox1, 'Max',2,'Min',0)  %allow multi selection
 list=handles.listbox1;                  
 temp = get(list, 'String');
 selected = temp(get(list, 'Value'));
 set(list, 'Value', []);    %Clear current selection for next time

I then use the following to perform the sum and median combine:

l=length(selected)
path=getappdata(0,'Folder');   %Get the folder 

imSum=[];       %initialise
imStack=[];
for i=1:l
   file=fullfile(path,selected{i})
   im=double(imread(file));
   imSum=imSum+im;
   imStack=cat(3,imMed,im)
end

   imMed=median(ImgStack,3);

However, I get an error:

Error using  + 
Matrix dimensions must agree.

Error in MultiImageanalysis>pushbuttonAddImages_Callback (line 1372)
   imSum=imSum+im;

Answer 1

[Jan]

The error means, that the image files have different dimensions. Either they have different sizes or some are true-color, while others are indexed images.

I'd start with:

imSum = 0;

[Jason]

Thanks