I think @tohster answer covers photo usage quite well. So my answer will expand on the subject from a different angle. There are numerous examples of profile photos used within ERP and enterprise solutions in general. For example: Microsoft outlook uses senders photos in email headers and that looks like the mock-up below:
Another example could be found in team communication tool slack:
So the usage of photos in ERP and enterprise solutions is actually common practice. This being said the pros and cons need to be weighed carefully before opting to use profile photos.
Based on the way you framed your question,particularly when you say that your app aims to "help users get things done" and that "There is quite a bit of interaction in there among users, but it's not a social app" it occurs to me that you have the basic ingredients for a gamified process.
Gamification aims to encourage higher levels of participation in tasks, and rewards consistent contributions from participants by tapping directly into our emotions and our instinctive need to have FUN. Simply put teamwork, recognition, problem-solving and collaboration are Fun and profile photos or any other imagery provided instead like "avatars" are key elements in this gamification process.
Enterprises are just beginning to realise the potential of applying game thinking and and game mechanics to business processes. So, allowing users/employees to let some personality shine through is definitely great.
Below is an example of profile photos used in a call centre environment where the aim is to increase call centre employee performance.
I guess overall, using profile photos will largely depend on paramteres you only know like the size of target audience, purpose and nature of collaborative tasks, visibility of participants and visibility of progress towards task completion etc.