The ISPRS data set collection
... always 
PS: If you are planning to use this dataset, PLEASE send us a short e-mail. We wish to maintaining a list of users to better facilitate future exchange of results and ideas. This will also allow us to notify you of any corrections or updates. Finally, please let us know of any errors or inconsistencies in the data, or if there is something you feel should be included. Contact: markus.englich@ifp.uni-stuttgart.de
Other image data sets
IGN - Geospatial and Terrestrial Imagery
Vision and Autonomous Systems Center's Image Database
The Ohio State University - Signal Analysis and Machine Perception Lab
Computer Vision Test Images