Not sure if you are looking for this answer.
I assume that you have both the high and low resolution images ready. If so go ahead and use the code
<a href="PATH\HIGH_RES_IMAGE_NAME">Download High resolution image</a>
and similarly another <a> tag for low resolution image.
You got to change the Path and image file name in the above code that relates to your setup.
Hope that helps.
- Vijay G
Strive for Perfection