What should filenames and URLs of images contain for SEO benefit?
From our (Google's) point of view, you can use whatever file names & URL structure that makes sense for your site -- you definitely do not need to fine-tune it on this level for SEO purposes. For Image Search, we recommend using descriptive file names, but even if it's just a number (for example, when a photographer uploads files without modifying the file name the camera used), we can usually work with that just fine - we use a lot of signals to pick up information about an image.
The only thing I would recommend keeping an eye on with images in particular is that your chosen URLs end in a common image file extension. That makes it easier for us to recognize them as images (before crawling).
SEO tricks are no longer relevant. Also google discurages those tricks, focusing more on authenticity.
So if you want to increase in rank, focus on content, have an adiacent blog to your site, go on social media. I think images, files, domains and URLs should obviously have relevant names and alternatives, but the main focus is the content, and you being referencing by other sites and social media.
In my opinion, image file names are one of the most important SEO factors... as long as it's valid and meaningful.
Don't take the same exact image and rename it over and over.
Don't give an image a deceptive file name.
Let's say I have the following images on a website:
black-and-white-dog.jpgfriendly-orange-male-cat.jpgafrican-grey-parrot-77-years-old.jpgferret-smiling-smuggly-2017.jpg
If those files are pictures of what their file name implies (and are original/unique) then it's a very strong indicator that the site is about pets/animals.
If the file names, photo content, and site aren't about pets/animals then you're in trouble.
Review this for background info:
https://research.googleblog.com/2014/09/building-deeper-understanding-of-images.html