Well I could add a link in each comment "move to top" like there is a link "delete" or "edit", but the reasons why there isn't any are:
- I didn't take time for making that yet
- I am still not sure that it is a good idea to make that: images of comments moved to top are not backuped with other newly posted images, as they are considered as "old" images. Though that it is the same problem with images added in old comments.