Depending on the resolution (roughly speaking, the quality) of the picture enlarging it may not be enough to recognise the faces -- you'd just get a blurry image.
There is, technically, no such thing as an HTML image, it's either gif, jpeg or PNG (or occasionally others). So presumably you mean the image as you see it on the link sent to you. Right-click on the image, select "Save Picture as" and save it on to your computer as a jpeg or (gif or PNG, etc.) image, and then you shoudl be able to open the image on your computer somewhere e.g. paint, or Picture and Fax viewer.
When you've opened it there you should be able to zoom in relatively easily. If the image is too blurry even when zoomed in, you might want to consider finding someone who knows more about computers than I do who may be able to do some sort of image-enhancing job or other.