Resizing Image Height

Thank you for your responses! Unfortunately they didn't work (I realize my question was a bit hard to answer given that I didn't provide any example code), but I was able to find a solution after pouring through StackOverFlow.

In order to resize the image, to fit within the li, I did the following:

First, I added a "gallery_pic" class to my gallery images.

<img src="img/RainySunset.jpg" class="gallery_pic" alt"">

Then I wrote a bit of code for the "gallery_pic" class to resize the image.

.gallery_pic {
  object-fit: cover;  /* Scales the image */
  object-position: center;  /* Center the image within the element */
  width: 100%;  /* Fills the li horizontally */
  height: 80%;  /* Fills the li almost all the way vertically, but still leaves room for the caption text below it */
}

Lastly, and MOST IMPORTANTLY, you have to set the height for the li

#gallery li{
    float: left;
    width: 45%;
    height: 300px; /* <--SUPER IMPORTANT! I set mine to 300px but you can set it to pretty much whatever you want */
    margin: 2.5%;
    background-color: #f5f5f5;
    color: #bdc3c7;
}

I think that's everything! Just a side note, Workspaces will highlight object-fit and object-position red. I think this means Workspaces doesn't recognize it? But don't worry about it, the code is valid and the browser knows what to do with it!

Thanks again to everyone who responded and was willing to help me :)