Replacing elements in a matrix in an if-else statement?

I have an assignment where I am supposed to make a function that will take a photo of myself, with a sharply contrasting background, pick out the image of me, and place it into another image (mine is of Pluto) like a fake tourist photo. This is my code so far:

stub=pluto(151:end,52:111,:);
for i=1:numel(stub)
               if me(i) >= 229;
                    stub(i)=stub(i);
               else
                    stub(i)=me(i);
               end     
end

The "stub" is a piece of the Pluto image that is the same size as my photo, so I was attempting to pick out the threshold of my photo with the if statement and replace its values with my photo. The background of my photo is a bright green, hence the 229 value. I have also tried this code with a for loop through every row, column, and 3rd dimension, and tried to pick out only the value in the green layer, but for that attempt and the code above I keep getting strange results that are not what I need. Which is the better approach, and what is it that I'm doing wrong?

Thanks!

[Chad Greene]

This sounds like a fun project. I have a feeling the loop and the if statement are both unnecessary, but just to check, I need to ask,

1. It looks like your Pluto image is RGB because it has a third dimension. Is that true?

2. Is the photo of you RGB or grayscale?

3. Assuming the photo of you is RGB, what are the RGB values of the background?

[Janell Lopez]

Both are RGB. It's a beginner's class, so I think the for loop is implied, but if there's another way I'm all ears! The value of the background is a bright green, with R=181, G=229, B=29.