Any image they created has automatic copyright, so if you use it without their permission, they can sue you for copyright infringement. They can also demand that you stop using the photographs (as the images are their property), and if you don't, they can seek a court order to force you to do so.
As by law, photographs, illustrations, and other images will generally be protected by copyright as artistic works. This means that you, as a user will usually need the permission of the copyright owner(s) if you want to perform certain acts, such as copying the image or sharing it on the internet. References to “images” in this Copyright Notice include:
- digital photos were taken on mobile phones and digital cameras;
- images that were first generated on photographic film and any digital images created from them; and
- images such as diagrams and illustrations
The amount they will propose is contingent on the nature of the copyright infringement, whether you were aware of it, whether it was a one-time or repeated occurrence if you have profited from their picture, and whether they have lost money as a result of your use, and other criteria.
In this regard, you'd have to respond to them yourself or hire a law firm to do so. You might apologise and explain that you were uninformed, that you did not make any profit from it, and that you would like them to drop the matter.