If it were me, I would set up a meeting (or send an e-mail if a meeting is difficult) with the person who is asking for folks to volunteer. I would want to cover three things:
- 'Some people' are reluctant to volunteer
- They have concerns with the broad language in the form
- A few changes to the language will make more people willing to participate
In true Dale Carnegie Fashion, you are speaking to the other person's interests, showing sympathy for their goal, and being helpful in trying to suggest a way to fix it.
Yes, you are the one who doesn't want to sign. But there are (presumably) others too. Don't make this personal, but rather be sure to present it in the interests of the person asking for volunteers:
"I really think it's a great idea having photos of us doing what we do as promotional materials, and want to see it happen. Unfortunately, some people seem to be reluctant to sign up right now."
Pick up two or three main points that you think are particularly problematic. For instance, if the right to use them lasts forever you can say:
"For instance, here it says we give the right for you guys to use our photo on promotional materials forever. The concern I heard is that our photo could be used even if we end up in another company in the future."
Try to keep the examples short, relevant, and practical. If you have to stretch for an explanation, don't include it.
Here's how to fix it
After you've explained what the main objections are, provide a commented version (even by hand is fine) with the suggestions you would make to have it be a better agreement, or with the troublesome parts highlighted. Just say something like:
"I'm no lawyer, but I jotted down some quick comments on the agreement based on the concerns I've heard of. You don't have to implement them all, but it may improve your chances if you had legal take a look over them and make it easier to sign. Thanks for your time!"
At the end of the day if they don't want to do that (because they don't want to keep track of who is still employed and whose photo they can/can't use, or because they don't want to have a costly reprinting job whenever anyone changes jobs, etc.), just don't sign it. It is voluntary after all. But if the person who is listening cares, you'll have a much better chance of it getting implement if they're the one championing it, since they're ultimately responsible for the results.