I can explain an overview of the process but can't get into too much detail without knowing the program. The approach to the process might vary based on the image you were working with. Here is the approach I take.
1. Make sure the original image you are starting with is on a editable layer, if its a background layer, double-click it to unlock it and give it a name. Keep the Layers palette open for future reference.
2. Take the image you are going to use as the replacement background and add it to the original drawing as a new layer. There are many different ways of doing this but I would probably just suggest using Select > All and then Edit > Copy in the replacement background image, then go to the original image and do Edit > Paste. Note the change in the Layers palette now showing both images. Close the replacement background image as we will only need the original image for the rest of the process.
3. Set the correct layer order, next we'll want to change the layer order so the original image is directly above the replacement background that you copy/pasted in the prior step. Drag replacement background layer to the bottom of the list. Note: this is not possible if you didn't complete step one.
4. Last step would be changing the original background in the original image to be transparent to show through to the replacement background below. This would involve making a selection of the original background and then making it transparent. The quality of your selection will impact the quality of your results. Once again, many different ways to make the selection, the recommended process would vary based on source image and which application you were working with. Generally, I would use the Quick Selection tool and then Refine Edge and use this as the basis for a Layer Mask. You could just as easily make a selection with the magic wand and just delete the content though. I would recommend looking around the net at different tutorials. You'll see may different approaches but all using more less the same overall process described here.