It sounds like you've been doing the search in the menubar Spotlight field. If so, here are two things to try:
One, when you have located the image you think you want to open, hit Command-Return, which will open (in a new Finder window) the folder in which the image file is located, even if it is inside your iPhoto library. The default application for opening the image then should be Preview (it is for me).
Two, select "Show All" at the top of the results, which opens a Finder search window. An image file in the resulting list, even if it is inside the iPhoto package, will open by default in Preview (That's what it does for me.)
Hope this helps.