Per the ePub standard at http://idpf.org/epub/20/spec/OPS_2.0.1_draft.htm:
"2.3.4: img Element
The inline element img should only be used to refer to images with OPS Core Media Types of GIF (http://www.w3.org/Graphics/GIF/spec-gif89a.txt
), PNG (RFC 2083), JPG/JFIF (http://www.w3.org/Graphics/JPEG
) or SVG (http://www.w3.org/TR/SVG11/
). The required URI attribute, src, is used to reference the image resource, which must be listed in the Manifest.
The required alt attribute should contain a brief and informative textual description of the image. This text may be used by Reading Systems as an alternative to, or in addition to, displaying the image. The text is also an acceptable fallback for an img with src referencing a non-OPS Core Media Type for which no viable fallback was found in the manifest. The alt textual description is useful for Reading Systems having limited resolution displays, or for non-visual presentation. Use of the object element is the preferred mechanism for including non-core media types in an OPS Content Document.
For long descriptions, the optional title attribute may be used. Reading Systems may display the title attribute in addition to or in place of the alt text or displaying the image.
For greater accessibility, it is strongly recommended that OPS Content Document authors include a URI reference in the optional longdesc attribute referencing a resource (such as another OPS Content Document in the Publication) describing the image in finer detail. Reading System developers are also strongly urged to recognize and render in an appropriate fashion (and with accessibility in mind) the resource specified in longdesc. For further information on the use of this attribute and related accessibility attributes, see http://www.w3.org/TR/1999/WAI-WEBCONTENT-19990505/#gl-provide-equivalents."