You can add cover art, an image that displays in the body of the bib record or links to another website. If you look at "Bio of a Librarian" in the public catalog you will see my test bib record that has all of those.
The images themselves must be hosted somewhere online so that you can get a link directly to the image. Like on your city/library website, or even facebook if you select the image and right click on it and then click on "copy image address".
Once you have the link available you need to add the link to the marc record using the example below from the "Bio of a librarian"
The first line will display the image as the cover art (the picture of me) - we have discovered that cover art added this way will NOT display in the new carousels.
The second line is not an image but would be a link to another website for additional information.
The third line adds the image in the description of the item in the public catalog (the ALS Logo).
Make sure the indicators are as shown - they don't necessarily mean what catalogers expect but they are required for enterprise to display them correctly. Adding the "z" field also adds the link in the description of the item by "Electronic Access", it isn't necessary for the cover art but used for training purposes here.