How to create 'mock-up' picture?
 

Hi,

I've seen some member's with photos of them before/after losing weight that look like they have been computer generated.. How do I create one?

you'll have to use a photo editing software like Photoshop or Paintbrush (windows). Basically, you'll open the two photos you want to use, then open a blank page equal to the size of both photos side-by-side.
Cut the photo from the first image & paste on to the blank page, then cut the photo from the second image & paste it to the other side of the blank image. Use the text tool to create the before & after text.

OR you can google "how to create before & after photo" and there are plenty of online sites that will do it for you once you upload your photos.