Slider Picture Size in Customizr

I'm brand new to WP and this is my first time using WP or Customizr.

I've tried uploading several pictures with several different pixel ratios for my slider using several different slider size heights with no luck. I've run into one of two issues over at least 20 different iterations.

First, when I use the larger pixel size (higher quality, original image), the image is cut off so only some of it is displayed on the slider (but at least it's not pixalated). I tried to remedy this by shrinking the photo (many different sizes including the recommended 1170 H), but it didn't help. It instead made the picture grainy.

Second, I changed the height of the slider, but the higher I made the slider, the more zoomed in the picture became to the point where it makes no sense to have a picture.

Is there anyway to use a high quality image, but shrink it (or change the slider height) to make it fit? Also, I read a few different things about only changing the height of the photo and not the width, but I don't have that option using Microsoft Paint (I believe it's because the picture is square).