Upload & Sell: On
How do you do this? Do you need Photoshop?
It's a menu option on the 5D3.
For other cameras, you would have to do it in any image editing program that has layers, or an astrophoto image processing program (Iris, ImagesPlus, Registax, MaxDSLR, Astrostack, DeepSkyStacker aka DSS, MaxIm, Nebulosity, K3CCDTools, Pixinsight etc etc)