You can customize the background images for your login page.

First go to Settings - Platform Settings

Now under the "Carousel and Login Images" tab. Check the images you want to show for your login page background images. The images will be randomly rotated through if you have selected more than one. Make sure you have at least one selected otherwise it will rotate through the default ones.

Make sure you click save after making changes

If you want to upload your own images, click the "Add more images" button. Select your images and wait for them to upload.

After they've uploaded repeat the first few steps and then save your changes.