For a single, free ROM file, the actual risk of legal action against an individual user is near zero. Nintendo typically targets large ROM aggregators, not hobbyists. However, you face other risks:
Released in 2008 (Japan) and 2009 (North America/Europe), Flipnote Studio (known in Japan as Moving Notepad ) was a free downloadable application from the Nintendo DSi Shop. The DSi was Nintendo’s first handheld with two cameras and an SD card slot, and Flipnote leveraged both. Flipnote Studio Dsi Rom