A single still isn't an image stream, and shouldn't be imported as such. Import image stream is a function with a very specific use, which imports each image of a sequentially numbered series (an image stream) as a single frame and converts the entire series into a video clip. If you use it to import a single image, you will end up with a video clip with a duration of one frame.
The correct way to import a single image, video clip, or any other individual piece of media, is to use the Import Media command. Then you will be able to adjust the duration of the still image as you like.
The knowledgebase article How do I import images?
discusses the specific details of this as well.