How much storage do photos take up? I am considering asking my parents for an iPhone 7 or 8 for Christmas, but I can't decide between which. 

Straight Talk offers the iPhone 7 with 32 GB and the iPhone 8 with 64 GB.  I'm not planning on downloading any apps on my phone.  How much storage does each photo (all options, resolution, live or not, etc.) use?
The size of the photos will depend on the camera and the size of the images. I have photos on my Mac that run from a few hundred kB to around 4 MB. My backup folder for my Photos has more than 2000 photos and takes up 2.5 GB but I have no movies. I would check the difference in the size of the cameras in the 2 phones. They are probably the same but I don't recall. A 12 MP camera will take images that require more size than a 5 MP camera.

That being said, while 32 GB is probably fine for you, I would opt for the 64 GB option. You never know what your interests will be in the future, and you can't increase the size of your memory on an iPhone. You may decide you like to make movies with your iPhone which takes up much more space (think 30 images per sec for a 10-sec movie) or you decide you want to edit your photos or movies which could mean downloading apps. The 64 GB option allows your iPhone content to grow in more areas. This is just my opinion.