[NTG-context] External Figure add caption or alternative text for tagged pdf/x

Hi All. I am trying to embed an image in a pdf so that when you open the pdf in acrobat or another reader the image is correctly tagged as an image and there is alternative text. So far I am able to export the back end tags so that I can see tags on tables and paragraphs, but the image comes through tagged as a paragraph and I am unable to add any alternative text. This is the code I am using: \setupbackend [format=PDF/X-1a:2001, export=yes] \externalfigure[district.pdf][height=10cm] Does anyone have any idea how I could fix this? Thanks! Claire