Use "alt text" to provide an alternative way for a screen-reader user to know what an image depicts.
The rationale for using an image is important for determining what the alt text should be.
Specific use: If you are using the image for a distinct purpose, the alt text should describe the image well. For example, if you were using the photo above for a botany class, you might write alt text that reads "Hydrangeas, which thrive best with full sun in the morning."
Decorative use: If you are using the image for illustration, the alt text can be simple. For instance, with the photo above, you could enter "Flowers." Some platforms will provide a checkbox for you to note that the image is merely decorative, thus requiring no alt text.
Placing your cursor on an image allows you to access the Image Options to create alt text. There, you can enter a short or longer description or check the box next to Decorative Image.
See Manage Alt Text for Images Embedded in the Rich Content Editor (Canvas) for an illustrated guide.
Right-clicking an image gives you a menu where you can add or edit alt text.
See Add Alternative Text (MS Word) for an illustrated guide.