He had a talented artist make nude posters of Chrom and distribute them to every tent

Robin could hardly believe it when she found out an hour ago, how had Frederick thought of such a brilliant idea before she did?
The posters he'd had made look so good. Her Master in a bold pose: naked, save a scale in one hand and a sword in the other. And at his feet, scrawled a new recruiting motto: "Chrom Wants You!"

Robin realised Chrom wouldn't want this as soon as she saw the posters Frederick was delivering to the tents of every member of the Shepards and their entourage. So she did the only sensible thing. She followed him and removed the posters from the tents of all the men and on the posters in the tents of the women she put a little magic that would make is so they would only tell each other about it and none of the men, least of all Chrom himself. In the tents of the men she replaced the posters with ones where Chrom is wearing a mirror polished set of armor and looking positively dashing.

"I had an artisan create posters emblazoned with your noble image. It's milord in a bold pose—naked, save a scale in one hand and a sword in the other. And at your feet, I scrawled our new recruiting motto: "Chrom Wants You!" I had them pinned inside each and every tent. Surely the troops will be thrilled to rally behind their common leader, milord." Frederick enthusiastically tells Chrom who flushes red in embarrassment and looks on in horror. Before he can chastise Frederick, Robin puts a hand on his shoulder, looks at him with a reassuring smile and says "Worry not Milord. I caught Frederick with his flyers and removed them. I had them replaced with more sensible ones, like this one."

Seeing the poster Robin shows him, Chrom lets out a sigh of relief. He is surprised that Robin, mischievous and libidinous as she is now, was the one to make this sensible move. Almost suspiciously sensible, but despite her new genie nature she is still acting mostly like herself so he sets that suspicion aside for now.