Tumblr/Website Design Gurus: What is the code that allows me to post only certain size images?

For instance, I’m wanting a max height and width per horizontal and vertical image that I post. How do I keep the scale/ratio the same but insert a code that will allow only a max height and width?

Currently I have an image that is posted at 960 × 1051 but I want to essentially cut that in half. I believe the 960 × 1051 is the original upload size and is displayed on my website that way but looking for that code that will automatically scale and keep ratios the same, while keeping the website scroll flow proportion equal.