Many people don't like their photos taken. It isn't surprising you are worried one or more of your guests won't be excited to participate. I recommend telling people in advance, so they have plenty of time to plan their outfits and looks. Much of the uneasiness about photos comes from not feeling prepared. The best way to drum up excitement the day of is to be excited yourself. Some emotions are contagious, and if you are having a great time, others will likely feel the same. Plus, that authentic emotion will come through in the photos.