When it comes to developing WordPress Themes that you want to add to the wordpress repository, you will need for sure some placeholder images that are GPL compatible. This can turn into  a major headache because wordpress rules are very drastic when it comes to licenses  and all the assets you are using, including images/photos.
Luckily now there are some places where you can download such images, that are released under the WTFPL license (which is GPL compatible). You can download this images from the following sources:
WTFPL Images on Flickr
It’s a collection of nice images, pulled together by Christine Rondeau that you can download at high resolutions and use them in any way you want.
WTFPL Images on Wikimedia
Also a cool collection of images and cliparts that are free to use in any wordpress theme or any other project you desire.
Happy downloading!