1. Check if the image really needs a description. Maybe it is decorative. The W3C alt decision tree(opens in a new tab) is an excellent resource to help you categorize your photos.
2. Avoid writing "Image of", "Icon of", or "Picture of". Screen readers announce the presence of an image. Adding these words inside your alt is unnecessary.
3. Keep it as short as possible. Be specific but not overly descriptive.
4. Try to put the most essential information at the beginning of your description.