Megan Fox Photo with Kids Spurs Legal Letter to Robby Starbuck from P

The guy who accused Megan Fox of child abuse for letting her kids wear whatever they want is now getting an earful from the paparazzi agency that owns the pic he used to call the actress out ... and they're not happy.

Backgrid -- the agency that owns the rights to the photo in question, showing Megan with her three sons -- fired off a legal letter Tuesday to ex-GOP congressional candidate Robby Starbuck who used the pic this past weekend to claim MF was forcing girl clothes onto her boys.

The correspondence, obtained by TMZ, accuses Starbuck of infringing on their copyright to the picture -- which they say Starbuck did not purchase or license before he published it.

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Megan Fox Wages War Against Guy Who Accused Her Of Child Abuse

Backgrid shuts down any potential claim to "fair use" ... a legal doctrine that allows third parties to use copyrighted works without permission in certain limited circumstances.

They say Starbuck can't argue the pic was for political purposes, which was clearly his intent here, and does not meet the legal standard to qualify as fair use ... on multiple fronts, in fact. Bottom line -- they say his fair use claim ain't flying.

Backgrid is now demanding Robby comply with their demands -- saying the photo he posted has been viewed by millions of people, without them seeing a dime -- or else.

They take one final jab at him -- and, presumably, Trump -- with this ... "While historically not always the case, we expect those who hope to serve as our elected representatives to lead by example and fully comply with federal law. Today, we demand you do so."