The trouble with putting these things in the BTS section is that 1. there's usually not enough space and 2. the above articles would be improved by having images. We are trying to make the best possible guide to Doctor Who, aren't we? If the BBC say that a certain illustration is what a character looks like, we are creating an information barrier for readers by not putting it in the main section of the article.
You say illustrations for stories can be argued to be a part of stories, I disagree. A book begins with the prologue and ends with the epilogue. An audio starts with the opening them and ends with the closing theme.
Has an image of Flip Jackson ever appeared in a story. No, it appears in the packaging for the story. An image file that is associated with the audio files for the story, a teaser of next month's release, interviews with the cast, and a music suite. The cover of a booklet on the making of the story that goes in front of the CD.
Would you ever write a sentence outside of the BTS section that uses information from the cover? "Although Evelyn said the Doctor was wearing his colourful coat, he was actually wearing his blue one." "Despite the fact that he called himself a bat, Professor Baffle was actually a bird puppet." "The Eighth Doctor stood calmly while he was roasted alive" "The only people that ever saw this flower were colourblind and thought it was white instead of red"
Covers flesh out things from their stories, but if they contained a picture of a newspaper article with information not seen in the story, would you be fine with pages being created based off of only the cover? What if it was the cover to a DVD?
The trouble is, Walton Winkle is a better page because of it. I don't want us to start listing the Tenth Doctor's intelligence level based off of his DWBIT collectable card. I don't want us to use that one puzzle from the mid-70s to say that an army of K1s once stormed a planet. I want us to be able to improve our articles by having images of characters that don't have much images.