Guideline
Every image should provide alternate text. However, if the image does not provide important information and should be ignored by screen readers, the alternate text should be the empty string.
WCAG References
Refer to the following topics for more information:
H36: Using alt attributes on images used as submit buttons (http://www.w3.org/TR/2008/NOTE-WCAG20-TECHS-20081211/H36)
H37: Using alt attributes on img elements (http://www.w3.org/TR/2008/NOTE-WCAG20-TECHS-20081211/H37)
ePortal Recommendations
Editing Client Messages
If you set the ControlStyle property for a particular field in a client message to Image, then ePortal uses the Label property to generate the alternate text for that image. In this case, set Label to an empty string if the image should be ignored by screen readers, or set Label to a meaningful text description of the image.
Refer to Creating and Modifying Client Messages for more information.
Manually Editing Web Pages
If you manually add an image to a web page, then you should set the appropriate alternate text manually:
For an MVC application, set the alt attribute on the img and input type=image elements. The alt attribute should never be omitted; set it to the empty string if the image should be ignored by screen readers.
For a Web Forms application:
If the image should be ignored by screen readers, set the property GenerateEmptyAlternateText to true on the asp:image control.
Otherwise, set the property AlternateText to an appropriate string on the asp:image control.
Note: As a precaution, you can always set GenerateEmptyAlternateText since it is only relevant if AlternateText is the empty string.