I haven't come across this bug, but I have written forms in the manner you describe, using image inputs with values according to the action being taken. If I recall correctly, the names have the X and Y values of where the image was clicked, and you need to parse the field names to find the name of the field clicked.

I think the values are something like submit.x=3 and submit.y=12. If you can, change the script to iterate all the form values and see what is being sent, or change it to a GET request.

edit: I think this post has some detail which might be useful:
http://lists.evolt.org/archive/Week-of-Mon-20041108/166064.html

Gareth