The website LittleThings is suing the rapper for posting its picture on his Instagram to promote his new single, "Vitamin D."

In docs, obtained by TMZ, LittleThings says it published an article back in March titled, "Is Boob Sweat Normal? 8 Things Women Should Know About Sweating 'Under There.'" The article included the cartoon showing a woman applying deodorant under her boobs.

Two months later, Luda posted the image with the text ... "IT'S THAT TIME OF THE YEAR." LittleThings says its logo is visible in the lower right-hand corner of Luda's IG post ... which included an iTunes link to his new song, "Vitamin D." The post had more than 22k likes before it was deleted.

LittleThings is suing for damages and for a cut of any profits made off its pic.

We've reached out to Ludacris, so far no word back.