To me, mental grit means taking on hard-to-achieve tasks and goals as personal challenges to overcome. What images does the phrase conjure up for you?
Adventure, fitness and sports also come to mind, and I'd love to see unique images related to those three areas that represent the idea of mental grit (one drop of sweat dripping off a brow, or courage in the facial expression of someone who's just getting into shape, for example). Anything goes, but we're looking for emotive shots of people or more abstract concepts. Feel free to depart completely from the sample photos! Be wary of "hero" shots (a beautiful person with a perfect physique breathlessly crossing the finish line, etc.) unless you're putting a new twist on them. Have fun! I'm working on a start-up, and the photos could be used for our Web site, social media sites, sales collateral, other marketing, internal media, etc.