All images must have alt tags so they are machine readable. Trivial images such as those used for design layouts, do not need alt tags.
In T4, the description of a media item is pulled into the alt tag if the content type is coded correctly. If the description is not pulled in automatically, please consult with Web Services to fix the content type.
Text on images
If you can refrain from adding text directly on an image, please do so. If you must put text on an image, then the content on the image must be put into the alt tag so it is machine readable.
For example, the image below needs the content on the image added to the image alt tag to be accessible.
Images in social media
All images that include relevant text and posted in social media need to have that text in the alt tag or needs to be posted as text in addition to the image so that the content can be consumed by a screen reader.