I'm not holding out much hope that Rose will appear on the cover of the 10th Doctor Sourcebook either. The only sourcebook to feature a companion on the cover has been the 1st Doctor, and I wouldn't be surprised if that was only because of the lack of high quality colour photos to choose from.
I may be wrong but perhaps Rose's (sad) absence from the cover is due to some special contractual clause - when they reprinted the various 9th Doctor / 10th Doctor BBC novels, they had to remove her from the cover. This also applies to audiobook versions of the novels (which were produced thereafter). IIRC, Billie Piper's contract stipulated that she would not appear on the covers of products after her run in the show was over or something along those lines. So probably it's a similar thing here.
Here are a few examples of those cover modifications:
If you look back at the other covers, you'll see that the companions are rarely on there.
Yes - I had noticed that (wot? no Ace on the cover of the 7th Doctor sourcebook?? )
That being said, a cover is only a cover. And after a first, quick browse-through of my 9th Doctor subscriber's copy, I've noticed two things:
a) We do get a nice picture of Rose on the "pseudo-cover" of each chapter
and b) Looks like the folks at C7 have pulled out yet another exceptional book here. With (alas) only a single season to cover, there is a lot of detail on each individual episode and they've also addressed various nifty topics - such as the retroactive interpolation of the War Doctor between the 8th and the 9th Doctors or how to recycle 9th Doctor episodes in play despite the fact that they're likely to be far more familiar to most DWAITAS players (being "only" 10 years old, as opposed to those of the classic era). Once again, the specific spirit of the individual Doctor/era is deftly defined and covered - and once again, you can tell that this book was written by gamers for gamers (as opposed to, say, a splatbook adding stats to information you can easily find on the web or in other sources). IMHO, this is what makes the Doctor sourcebooks so outstanding so far - this emphasis on practical GMing and actually running the game. Judging from the first eight books, I'd also say that they've managed to treat the "short-run" Doctors as expertly as the "long-run" ones, taking into account the particularities of each era and never falling into the pitfall of formulaic treatment (despite maintaining a consistent format throughout the series).