Here's more on the type of images you should be using in fundraising. Some research reported in the AMA Journal of Marketing Research -- The Face of Need: Facial Emotion Expression on Charity Advertisements (abstract; the full study requires a subscription).
Researchers found that sad faces prompt more giving:
... people are more sympathetic and give more to a charity when the victim portrayed on the advertisement expressed sadness than when a victim expressed happiness or neutral emotion.... the authors illustrate when and how a sad expression enhances sympathy and giving. Taken together, the findings imply the importance of subtle emotional cues that sway sympathy and giving.
We should approach studies like these with a heavy dose of skepticism; they aren't fundraising situations with real donors, but artificial situations with college students.
Also, there is a lot more information in a photo than a sad/neutral/happy expression. There are sad faces that make people feel defensive. There are happy faces that somehow go straight to the heart. Sometimes there are things in the background of the photo that undermine what you see in the faces.
I'm showing you this because it backs up what every experienced fundraiser already knows: Sad faces get more response.
But that's just a starting point. You have to look at every photo carefully before you use it.
Thanks to Advice for Good for the tip.