Storing images as nodes
The image module does not exist in Drupal 7 because this functionality is added to Drupal 7 core.

Implementing images as nodes was a clean way to do this in Drupal 7. You may still do this in Drupal 7. However, Drupal 7 has gone a long way and offers the entity alternative. At Drupal 6 'everything can be a node'. At Drupal 7, 'everything can be an entity'.
Nodes were meant to be used for publishing text. They come with a lot of functionality that you may find redundant in your use case. All this extra functionality can slow down your site if you have thousands or millions of entries.

I am not sure I understand why you should not do this. It was probably a suggestion and module specific. In Drupal 7 you can name your content type anything you want.

As I told you before, using entities for storing images is a better alternative, as far as the performance is concerned. But then you should implement your own method of commenting.


EDIT: In Drupal 8 there is some advances made in decoupling the comment module from node. See this issue.