Hi Penny,
The guidance on the Divi site says this:
The width of your fullwidth slider background image is always determined by the browser width. Based on standard screen sizes we recommend that your images are at least 1280px wide. But for larger monitors, a safer bet would be to use an image that is 1920px wide
What I would do is find one of the existing images in the Media library and check out the size of those images. This will ensure consistency between slider imagery.
Let me know how this helps you,
Rob