Help with Mobile Feature Images on my website to be the same as on Desktop version

I need somebody to help me with my website. The website is already finished but I need help with having the same size image from the desktop pull across onto my mobile version. Currently the image is zooming in a lot and only focusing on a small part of the image instead of the whole image as it is on desktop. I have had a friend install the Mobile Feature Image Plugin but we do not know how to use it correctly.
Can anyone help? There might be other small things I need help with on wordpress too.
Thanks