Techniques are described for importing and using virtual machine images in configured manners, such as by a virtual machine image importation service on behalf of clients. An image may be retrieved based on a client-provided location, and various characteristics of the image may be identified via application of multiple heuristic or other assessment tests to various aspects of the retrieved image such as a master boot record, filesystem, or directory structure of the image to determine a testing vector that is compared to identification vectors associated with known operating systems, in order to automatically determine a specific operating system installed on the image to be imported. Modifications may be made to drivers, configuration and system files of the retrieved image based on the identified operating system and other image characteristics.