If you are trying to make an InDesign file 508 compliant in its Export to PDF and have done all the standards on assigning an Alt tag to the image, checking "Create Tagged PDF", etc. but find the image graphic still does not carry over its alt tag from InDesign. You need to make sure you do not have any "effects" assigned to the photo or graphic.
While 508 will accept a drop shadow applied to the image, PDF will NOT read the tagged image-figure as an <Image> (but a <Path>) if the image has an inner shadow, or is has a transparency (i.e. 75% opacity), or has an effect of Multiply or Overlay (as to Normal). Turn off these effects and your alt tags should carry.
Hope this helps.
What are your PDF export settings? I would recommend to export as PDF (interactive)?