Start by applying pressure to any active bleeding wounds with a warm damp cloth, once the bleeding stops, trim the feathers around the wounded area, about a 2/3cm circumferential zone will work, after that lavage the wound with either saline, chlorhexidine (Hibiclens), or diluted betadine. You can use a dish soap bottle to lavage or a 35/65 syringe. Then you can either slather the wound with a triple antibiotic ointment, or use a poultry wound spray like Vetericyn.
Move the duckling into a warm, quiet, safe, stressful free area, with food and water provided. Some more clear pictures of the wound, once everything is cleaned up, would be good.
It's your call, if you feel its too bad and that she should be euthanized please do so, but all I'll say is birds are resilient creatures and have recovered from many severe injuries. If this was a bite wound though she'll likely need antibiotics, do you have any on hand? Do you see any break where her leg meets her body?