Why set an image's meta data?
Unlike humans, Google can't actually see the images on your website. Instead, they look at the written information surrounding your images, known as meta data.
Google looks at your images:
The name of your image, set in the 'Image Properties' which are accessible from the Editor or Classic editor when you right click on any image in your page.
Image file name:
This is the name of the file from when it was on your computer. For Example: my-image.png
The image's caption:
The caption is set on the web page and describes your image for users.
Alt tag on your website:
The alt tag shows if the image can't be loaded (maybe due to bad internet connection). It's also what's read out for those who use screen-readers to use websites.
Make sure each of the above not only descibe your image, but also include your keywords/phrases to help your SEO.