I would recommend using the UIActivityItemSource to provide metadata (such as the preview photo).
When the UIActivityViewController machinery requests the preview photo using the UIActivityItemSource's method activityViewController:thumbnailImageForActivityType:suggestedSize:
it will pass in a CGSize that you should make sure your image fits inside. Make sure to adjust the dimensions of the image to fit the suggestedSize if you want it to fill up the full width of the alert. See the AirDrop sample code for an example of how to do this.