If you want to add a background image, you should crop it down to be small enough, and the right size for the SNAP stage.
The ratio is 4:3 and the best pixel count is 480x360. This would also work for 960x720, but the file takes up more space and takes longer to load the bigger it is.
Read more about resolution and pixels here. And, specifically how to save in the best PPI to save quality of your image.
You will click on the Sprite you are going to add this image to, as a costume. Then choose the Import button under the drop down for File or DRAG it in.