Question : GIMP

I am using GIMP for the first time and am trying to replace one person (1) in a photograph (A) by another person (2), which I cut from another photograph (B). So I first prepared photograph (A) by cutting out person (1) and making the place where it had been look like its environment. This went fine. Then I had a lot of trouble copying person (2) from the other photograph (marking it by using "intelligent scissors" and "feather" etc.) and finally copied it into a new file (C). This is where my problem starts: Now I want to copy if from this new file (C) into photograph (A). But it always has a white background around it. I hope there is a possibility of just copying the person (2) from new file (C) without the white background, but I cannot find out how to do it, without having to cut it out again in a time-consuming operation. I assume there must be some way of copying it without the white background. Can anybody tell me how to do this?

Answer : GIMP

Could either of these be by any chance a .gif image?
Try right clicking on each image and go to image -> mode -> and select RGB if it is not already selected. It sounds like maybe you have one of the images saved as a .gif and it has a compressed color palette to save space that doesn't include transparency.