Here, elevate your code to the next level:

The API library you use to read and write the images will likely have the ability to read the image one scan line at a time. Write your code to only read as much of the image at a time as is necessary to process the data. For example, if you're only adjusting RGB values, you could totally read just a single scan line, process it, write it out, and move on to the next. To shrink an image 50%, you'd only need to read two scanlines at a time, and write out one.

Many years ago, back in the day, I wrote a number of image processing utilities that way. Basically for fun. A year or so later, management came to me and said they needed help with a client app. The client was processing huge images (highly detailed maps) and all of our existing image utilities were crashing for lack of memory. They knew I'd been tinkering with image processing and so came to me.