Image field in a form does not work on Mac

I created a PDF form with both open text fields and image upload fields.  I used a JavaScript-enabled button to allow the user to upload image files.  The instructions for using the image upload field were saved as an image file which was included as the default content of the button.  When the user clicks on the instructions as indicated, it launches the Select Icon dialog box.

This all works very well on my PC using Adobe Reader.  However I just had a colleague test it on his Mac and it did not work.  He first tried in Preview.  The entire image upload field did not appear at all, with the following open text field moved up in its place.  He then downloaded Adobe Reader for Mac and tried again.  This time the image upload field showed up as a blank square.  The default instructions image did not appear at all.  Clicking on the blank square did not launch the Select Icon box.

I need this form to work on both PC and Mac environments - how can I make it Mac compatible?

Thanks in advance.