If you want a profile photo to be mandatory, you better be getting a photographer in to take company staff photos. Then make it mandatory for them to use this photo as their Slack profile pic. Add it to your company policy and inform HR that this should be included in the onboarding process.

Downside: it's costly to have professional photos taken with each new staff member and some process need to be changed.

Upside: you're no longer talking to Calvin from Calvin and Hobbes. Also it looks professional and it's in my opinion better for overall company culture.