After importing photos from camera, iPhoto prompted to keep or delete photos. I clicked delete but then it crashed and now they're missing. How can I recover my photos???

First off that is a bad practice - never have any computer program delete things - import photos and keep them - after at least one successful backup cycle has taken place and you are sure your photos are safely on the Mac and backed up then use the camera's format command to erase and reformat the card

You can try to backup your iPhoto library and hold down the option and command keys while launching iPhoto - repair permissions and rebuild the database form the resulting first aid window - that may recover your photos - if it does not then photo recover software like media recover is the next step

LN