C# release memory from image, when assigning it to null

Question

I am making a video game. At this level, I need to find a way to release memory from images, once I set their source to null.

When I am not using an image, its source is set to null.

imgExample.Source = null;

But it doesn't free enough memory.

When I did similar thing to MediaElements, I used this approach to free unused memory:

        foreach (MediaElement thisMedia in Videos)
        {
            thisMedia.Source = null;
            thisMedia.Stop();
            thisMedia.Close();
        }

Then applied:

GC.Collect();

at relevant place.

Now, I am trying to do something similar with Images. The "Close" can't be used on Images. I tried using "Freeze()", but it only made worse. In any case, Freeze is used when assigning new source to Image, and I want the opposite - to do something when source becomes null.

I tried googling on the issue, and got to this page: https://msdn.microsoft.com/en-us/library/system.drawing.image(v=vs.110).aspx

I thought that methods: "Dispose" and "Finalize" can be useful.

However, I can't just use:

imgExample.Dispose();

or

imgExample.Finalize();

Either the approach is incorrect, or I am missing a using directive.

What is the correct way to dispose of unused memory?

EDIT: I forgot to say, that "imgExample" is an image defined in XAML. Preferably NOT to replace such images with new ones in code behind.