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Hi Simon, I will try to answer your questions, however someone with more knowledge may have better answers.
Q1/ If you simply copy the same image 20 times and try to stack it you will end up with a real mess or at best, the same image you started with. The reason we stack many images is to increas the SNR (signal to noise ratio). There are different types of noise, thermal noise can be removed from your pics by subtracting dark frames. Signal noise is actually a random phenomena and cannot be removed easily. Because it is random it shows up differently in each image. When we stack our images using say the median settings, your stacking program looks at the signal value of each pixel in each image and nominates the median value of all the images for each pixel. (median is basically half way between the maximum value and the minimum value). So in this way you remove some noise because the noise is in a different spot on each image. If you stack a heap of copies of the same image, the value of each pixel on each image will be exactly the same, so the median will be the same as the value of each pixel. Did that make sense?
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