Any image that denotes information requires an alternative description for non-visual devices. If an image is purely decorative and serves no content purpose it can have an alt text with two quotation marks(""). This will tell asssitive technology to skip the image and move onto the next content.
When inserting an image in Kentico, note that the Image/Edit dialog provides a field for the alt tag.
The text should describe the image or contain the actual text if there is text in the image. Note that if you insert an image through the Quickly Insert Image option it will automatically put the file name into the alt text field. This should be removed and replaced with readable text.
Screen readers usually say "Image of" or "Graphic of" before reading the alternate text.
Learn more about images