When inserting images to your posts or pages, you have the option to add an alt tag to the image. This is the text that will display if the image can't be loaded. It's important to add in an alt tag for all images, for two main reasons:
1. It helps search engines understand what the image (and post) is about, and therefore enhances your SEO.
2. It helps visually-impaired people understand the image. People who use screen-reading technology rely on the alt information to interpret what the image is.
You should be adding alt tags to all of your images as you insert them. However, if you didn't do this and now have a lot of images on your site that are missing alt tags, there are a few different ways to ensure that all of your images have alt attributes.
There are many plugins that will automatically add alt tags to your images based on parameters that you specify, such as the title of the page or image. The tradeoff is that you don't have direct control on what the alt text will be.
A popular free plugin of this kind is PB SEO Friendly Images.
If you want to make sure each alt tag is different and correct for the image, you could use an automated solution that would scan your entire site for missing alt tags. You can then look at each image and add alt tags manually.
The only tool that we are aware of that will crawl your entire site and generate such a report is Screaming Frog SEO Spider. It's not clear whether this feature is available in the free version of the tool, or only in the paid version. The paid version is a bit pricy, though!
If you're looking for a half-way point between these two options, you could consider doing the following:
This will make sure that your most popular posts and pages have hand-crafted alt tags, while ensuring that every image has some form of alt tag.