They are not 'photographs' in the normal way, they are a print out of the scan. Sorry, but I do not think you can get copies. If the hospital has a print out as well they might let you photocopy it, but they don't have to.
Depends on the equipment used modern ultrasound machines have digital storage so it is possible it is still availabe especially as many patient records are kept electronically the uploaded image would be within that file. Whether they will let you have a replacement is another matter.