Alt-text is a SEO tool that you use within WordPress in the form of photo description. The main function is to describe an image as though a viewer can get information of what the image may be, even though it may not load.
Here you can take the opportunity to use buzzwords/keywords in your description. The idea is that – if your description is good enough – search engines will go to your image first.
So, for instance, if you have a picture of Bob at the bar, do not put “Bob at bar,” because it’s too short and vague. On the other end, do not jam pack the description full of words. A bad description: “Bob, bar, beer, alcohol, restaurants.”
So, in the image above, you could put “flower.” Another (better) option is “iceberg rose.” The best description, however, is “Sun shining on iceberg rose in a garden.”
Feel free to email or comment any questions or suggestions, or for further reading materials.
Please refer to Moz.com for more information.