Adobe AIR 1.5 still does not support any data-urls. I have no
idea why, because webkit supports data-urls for a long time. There
was a bug report for this issue in the Adobe bug tracking system,
but it seems nothing has been done since that. There are other
webkit standard features missing from AIR without a good reason:
CSS Shadows, reflections, transitions, animation, SVG etc.
Write a feature request or file another bug report, probably
this will help sometime.
For now you will have to save the image to a file on disk and
load it with a file url. You can use the temp-file feature, so that
the images are deleted, when you application quits. Not quite
ideal, but the best you can do for now.
I hope Adobe will include all the standard webkit features in
future AIR releases, I don't know the reason for being so
conservative with some features.