You have to consider the use case of the default installation, since the Ubuntu people are trying to make is a distribution that is usefull for the average users, the default installation need to cover the need of the many. And what most people want to do with their picture is to classify them, do light weight retouching (for instance, adjust the brightness, remove red eyes) and then print the image or send it on the Internet. Surely, both Krita and GIMP can do it, but they have way more features than what is needed for most users. As Sven Neumann, one of the core GIMP developer, said GIMP is a high-end application for professionals, and so is Krita. Some might think that Krita has a GUI that is more friendly to the beginner, but that is not the problem, it is still packed with features that are of no interest to most users. And there are tools that are better suited to accomplish the task of those users, this is why a pictures collection management tool, such as F-Spot or Digikam is a much better choice, it covers the main usage of the majority of users. Even if Digikam is designed for professional photographer, I still think it scale nicely to average users. And when the user want to do more with images, he can just go to his favorite package manager, and install GIMP or Krita.