Include Keywords In The Filename
Use keywords in your image name. This is one of the best practices for image optimization. Do not include space and special characters in your image name as they will be encoded. Use hyphens to separate the keywords in the image name.
Add Alt Tags
Search engines cannot read an image, but they can read the alt attribute included in the image tag. Make sure that you include keywords in the alt tag of every image. Do not use the same keyword for all the images on the web page.
Use Relevant Anchor Text
If you are linking to another page on your site using an image, add an appropriate anchor text. Include keywords in your anchor text.
Reduce the Image Size
Make sure that the images are not heavy, but load quickly on the site. A slow loading image does not give the right impression about your site. You need to keep your site loading time to a bare minimum as it affects the ranking in Google to some extent.
Surround the Image with Text
It is not a good practice to congest a web page with too many images. Always add a little text along with your images, if only a few lines. This will give the search engines more information to crawl and index.
Name the Image Folder Appropriately
WordPress has a default folder called uploads, for saving all the uploaded images. You can change the name of the folder and make it keyword rich. Henceforth, every time you add an image to your WordPress blog, your image file name will be searchbot-friendly.
Include the Title Tag
Title tag helps you add meaningful information to the image. This is helpful particularly for a site containing a lot of images.
Add Images to Sitemaps
Google indexes billions of images every day. Help Google find the images in your site by adding them to the sitemap. You don’t need to create a new Sitemap; you can just add information on images to the Sitemap you already use.
WordPress – Image SEO Optimization