How to make a image taken with D5100 1080P HD?

I want to make an image I took with my Nikon D5100 a 1080P HD image with a resolution of at least 1920 x 1080px.

Are you saying that you shot video at Full HD, 1920 x 1080p

If so, you might be able to do a screen capture of one of the single frames, but do not expect a very high quality image.

If you look on page 49 of your user manual, you will see that there's no 1080p still option.

They are:
4928 x 3264,
3696 x 2448 and
2464 x 1632