The Box Set Creator downloads box set images directly to your computer as a PNG or a JPG.
If you've previously download the box set image as a PNG and need a JPG, one way to convert the image to JPG is to upload it in My Images in the Custom Creator, choose an Ad Size such as Instagram and then add the box set to the workspace. You can resize as needed and download as a jpg.
Another site you can use outside of Book Brush for a quick, free conversion is: https://png2jpg.com/