I guess it depends on what you mean by "quickly". The images are unpacked into the AssemblyCache under the user's profile.
I get the directory with something like this:
string dirpath = Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location),"Images")
And then you can use normal .NET file IO to access the files. And these would be the files in visual studio that have a build action of AddInContent.
You can also add files with a build action as
Embedded Resource and get it with
There might be other methods, but the two above are what I've used (I guess 3 of you count resources).