I have about 1100 images with agencies that are already indexed. I dont want to send them any more. However if i follow the yoast SEO to get greens on all the images will it be worth it if the images are already indexed with multiple agencies?

Also a couple of other questions
Should we be renaming the files from like img9987.jpg to businessman in suit.jpg?

Also does symbiostock add "alt text" to the images?

Im trying to learn seo. Don't know if anyone knows the answers to these questions.

The second question...

It does not matter what your file is called because symbiostock gives them a descriptive filensme based on title. Alt text is given to them too