Animoto is a website that you can use to create promotional-style videos of images, video-clips, and music. Upload your images and video-clips (or point to them if you keep them online on a photo managing site like Flickr, Facebook, or Picasa), add text if you want any and then choose your music, either by selecting something from the Animoto collection or by uploading your own track. Animoto then generates your video.
You can choose in which order you want your images and video clips to appear but not control the transition between them. As always when dealing with images and music online you should only use material for which you have the necessary rights.
The basic, free Animoto account lets you create videos of up to 30 seconds. A premium service is available for a subscription fee.