make one new picture from 2 existing

i have 2 jpg picture files and I want to make one new one picture or copy the picture from one file and place it next to the other picture in the other file.

for example i have a picture of a red car and a picture of a blue car. I'd like to end up with one picture contaning a red car and a blue car

If I create a third picture and copy and paste an existing picture into it, the quality of the new picture is reduced.

can you advise how I should do this?

thanks in advance