Like I said in my answer to your other question, you may wish to look at IMAQ for something like this. Another suggestion, which is also relevant to your other question, is to post your VI if you have a problem with it.
As for your question, you will probably need to write an algorithm for going over the pixel array from one of the pictures and replacing only some of the elements with elements from the other picture (let's say going over the 1D array and replacing every other element, which is probably easiest). I'm not sure how good it would look, but you can try it.
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