iphoto to photos: where are files stored?
The original images are stores in:

/Users/USERNAME/Pictures/Photos\ Library.photoslibrary/Masters/

If you're having trouble with that, I would suggest opening a Terminal and searching manually in your user directory:

find ~/ -name '.jpg' -o -name '.png' -o -name '*.tif' 

It find everything that ends in .jpg, .png or .tif in your /Users/USERNAME/