All images in your files need to have alternate text that describes the purpose and important information for those who cannot see the image. While we used to just consider this as an accessibility issue for people who use screen readers, there are additional uses for alt text: people with low internet bandwidth may turn off images to speed up page loads, descriptions of images will display if the image does not load because of broken links or technical issues, and these verbal descriptions will be included for those using the MP3 audio file created by Ally Alternate Formats. The fundamental purpose of Universal Design for Learning is to design for learner variability which then benefits more learners in ways that you may not have considered.
So, how can you make your alt text instructive? When you craft alt text for pictures, charts, and illustrations, consider the purpose of the image. Why did you select this visual? What do you want learners to take away from the image? What is important about it in relation to your learning outcomes? Include this in the alt text rather than just a detailed description of what is on the screen.
If you need help brainstorming alt text in this way, you can try the Gem Alt Text Generator that will create variations of alt text for an image you upload. See this in action with an example from a biology journal article: Add complex alt text with Gem Alt Text generator