The duty is yours, not the software's
Whoever publishes the listing carries the disclosure obligation. Not the editing tool, not the photographer who made the alteration, and not the MLS. That is worth stating plainly because most tools in this category are sold as though buying them discharges something. They do not. What a tool can do is make the obligation cheap to meet.
What a rule actually asks for
The common mistake is reading these rules as a labelling requirement. Several of them are not. §10140.8 asks for a conspicuous statement and a link to a page showing the original. Canopy MLS asks for the unaltered image to sit immediately before or after the edited one in the photo sequence. SDMLS says labelling alone is not sufficient and asks for the original to be uploaded adjacent to the altered image.
Three different rules, one shared demand: produce the original. A label on its own satisfies none of them.
Where we say we do not know
Ten major MLSs gate their rulebooks to members: Bright MLS, NWMLS, NTREIS, MetroList, Georgia MLS, FMLS, MLSListings, SABOR, Realtracs and REBNY. We have not read the current text of those, so those rows are marked unverified rather than filled in from a competitor's blog post. Several tools in this category publish per-MLS fine schedules that appear in no rulebook we can find. Being the accurate one is the entire point of this product.