Most of the games will have a game box icon. We stopped adding them a couple months ago in anticipation of launching a new version of the TM which will use a different form of boxart. It's just taking longer than expected with all the daily work and then trying to find time to completely re-write the Trainer Manager.
But, no, you can't add in box arts yourself. We might be able to fill in some of the empty ones though.
Thanks for the VERY quick response and for all the awesome work that you all do at CH. It's not a big issue at all - I can be patient and wait for version 2.0