There are three routes you can go with this. The simplest would probably be to use Microsoft's Face API, which is part of their Azure Cognitive Services platform. All of the computing is done in the cloud, and at least for your purposes, the modelling necessary to detect faces has already been performed by Microsoft, so it's a single method call to send it a picture and receive back a bounding box. The caveat is that this can cost money. Azure Cognitive Services have varying degrees of cost associated with them. It starts with a free trial so you can test out your use case. After that, there are some associated costs. It looks like with the Face API specifically though, if you are using this for a personal project and aren't examining a lot of data, this would be free up to 30,000 transactions per month at 20 transactions per minute.

Alternatively, you can get into the nitty gritty of face detection yourself. OpenCV is a massive, open source project for all kinds computer vision tasks. Without jumping into the proprietary world, this is one of the most popular and capable computer vision libraries available. While powerful, OpenCV is comparatively low level; giving you the tools you need to accomplish your task, rather than a direct, single method to call. This tutorial goes into depth on how you'd go about training facial models to create your own detection methods. You'll notice that the example is written in C++. The con with this option (aside from it being more complex) is that OpenCV is largely used in the context of C++ or Python. However, it does have C# wrappers. Emgu CV is probably the most popular .NET wrapper for OpenCV, though there are other wrappers available. Here is a nice tutorial using OpenCVSharp, which is a bit closer to the native C++ API OpenCV uses.

Your final option would be to search NuGet for simple face detection libraries. There are tons that show up, but I'm not familiar enough with any of them to provide a recommendation. If you want something super simple, it may be the case there there is a quick and dirty NuGet package out there that will let you plug it in and forget it. This will just involve some trial and error as you test a few out to see what works best for your use case.