Shown here is studio usage of three different postage stamps instead of revenue stamps.
Using postage stamps to pay revenue taxes was prohibited in the "Schedules of Stamp Duties" issued by the Office of Internal Revenue, but interestingly, none of the penalties proscribed by law specifically applied to this infraction.
There was a $100 penalty for "omitting a stamp in attempt to evade tax" and a $10 penalty for "failure to affix stamp". Arguably, a studio could be held liable for the latter penalty for using a postage stamp, but there is no evidence that anyone ever was.
In reality, the use of postage stamps was simply the result of law-abiding photographers attempting to remain so in the absence of a "proper stamp".
Given that photographs could not be subject to the enforcement mechanism Internal Revenue brought to bear on documentary taxes, where use of an improper stamp would actually nullify the legitimacy of a legal document, it is very likely that they simply did not bother to enforce the ban at all.
2¢ Black Jack ex Paul Wolff