Wordpress - help to fit a picture on full screen with 100% ratio

Hi everyone, I have a simple request. On a wordpress website i have, I'm using the template Meshable ([login to view URL]) and i'm inserting pictures on full screen. The problem is that pictures are adjusting 100% of the screen (ratio is not respected), so there are some distorsion. If the screen is more a 16/9 (like on dell laptops) the picture won't look nice. I'd like what/where i have to insert the code to have a picture inserted on 100% screen with a respected ratio (ratio 100%). I think that the best is to insert this code on the Editor>Template. So the deal is simple : be the first to tell me how to do that and be paid $30. if your tip is not good, the contract is cancelled and you're not paid. if the tip is good, you're paid and get a good review. FYI, i will oncly consider people with experience on Wordpress. Thanks for your time.