The main criteria for inclusion should be notability. That being said, to avoid clutter and confusion, we should note the following.
Illustrations for Doctor Who-related articles are typically taken from screen captures, publicity materials, magazine articles and websites. As such they're nearly all non-free and must only be used in conformance with the Non-free content criteria. Exceptions include pictures of people involved in the writing or production of Doctor Who and its spin-offs, which have been released under free licenses: licenses that allow third parties to modify images and use images commercially. Most non-free images currently in Doctor Who-related articles lack detailed or sufficient fair use rationales justifying their use in the articles. Such images are in danger of deletion; you can remedy this by writing better rationales.
To avoid the tricky issue of canonicity, we should identify the source of the information if it comes from anywhere else besides the television series. Televised episodes and specials (barring spoofs and specially-made episodes) are generally considered canon by the BBC and the vast majority of the Doctor Who fandom, while the Peter Cushing films of the 1960s are not. Everything else is of unclear canonicity, with opinion ranging from absolute canon to non-canon. In practice, anything from the televised stories need not be sourced or distinguished, although the relevant episodes should be referenced (with citations to the appropriate episodes or serials). We should also limit the non-television series material, where possible, to material officially licensed by the BBC. This excludes fan videos, but will include the Doctor Who Magazine comic strip, the Big Finish Productions audio plays and story collections, the Telos novellas and the Target Books, Virgin New Adventures and BBC Books novels and short stories. Book titles, like television serials, should be italicised, with individual episodes, chapters or short stories in quotation marks. Author credit should be given. The only exceptions to the "officially licensed" would be material from BBV, which is generally of a higher standard than typical fan productions. However, any notable creation may be included, but only if we are clear on its origin.
Some articles on what might be termed minor topics, e,g. monsters, villains, planets or devices which only appear in a single episode are better placed in a list. These are some of the lists available:
Articles can be de-merged if they grow very large. Some very minor characters are probably not worth writing about, and it would be preferable to write the plot synopsis of the episode in which they appear: for example, the article about the character Doctor Constantine redirects to the article about the episode "The Empty Child".