Images are often added to educational materials to make complex information easier to understand or to simply add visual interest. However, not everyone will be able to see the image (e.g., people with vision impairments, and those who have images turned off to save bandwidth). Also, sometimes images don't load.
Alternative text (alt text) is text added to an image to convey its meaning and purpose of text for anyone who can't see it. Alternative text is:
It's best practice not to include text only in images. Text outside of an image can be more easily customized by readers to make it easier to read. For example, someone with dyslexia may change the font to a dyslexia-friendly font, and a person with vision impairment may adjust the size of the font.
If you do include text in an image: