HTC Desire - where are my photos?
As you removed the sd-card, you could use a card reader to attach it to your PC, which makes a search a lot easier. Usually, photos are stored below a folder called dcim on the card. Depending on the camera app (I don't have a Desire, so I cannot check that myself), subfolders there are named differently. On my phone, it uses a subfolder called "Camera". But there should not be too much subfolders there, so it will be easy to find.

If that's not the place where your niece placed the "old images", but as your Desire insists they are (somewhere else) on the card, you can use your file manager (I don't know what OS you are running on your PC) to search for *.jpg on the card -- that might match a lot of other images, but your photos should be listed along as long as they are on the card.