This tutorial explains how to combine two separate images next to each other to create a new single image file (I was recently asked how you do this) using the free image editing program Paint.NET. (Please note: this program only works on Windows, not Mac)
The first step is to open one of your images in Paint.NET by clicking on File > Open and browsing the files on your computer.
The next step is to change your canvas size, so that you can fit your second image next to the first. Click on Image > Canvas Size and then double the width in the Print Size box.
Next, go to Layers > Import From File and then browse your computer for the other file.
The second image will open on top of the first image, but you can click and drag the image to place it beside the other image.
You can now save your image as a new jpeg file.