how to make the consistent image size for both iMac and mac pro(laptop) (Jasper template)

Hi guys, do you guys know any code to make the consistent image size for both iMac and mac pro(laptop)? 
Currently, the image on my website is set to the full bleed and it looks perfect on iMac 27". 
However, when I look at it on my laptop(mac pro 15"), the image and gif don't look the same as the ones on iMac.
I don't want the images to get changed based on the window size. Please help.
Thank you!

I have a similar problem-and want to code so that the image galleries show the original image size and are not re-sized or re-scaled for mobile...