In my opinion best "advertising" for Gimp is to do good art or useful image processing with it. Photoshop is mass market product with huge user base and a lot of (commercial) support from artists and image processing professionals. However, I'm not that convinced that all those pros want to support Adobe with their hard earned money, if they get what they need free.
No need to compete, just do good work with Gimp. Those people who only need 10% of the features of any proper graphic app would be very happy with Gimp. Those who need something like print-ready CMYK will probably not be willing to change from Photoshop anytime soon.
I'm really happy with Gimp, 2.7.2 is marvelous with all the new and improved features. Many great artists here and on other Gimp user sites have shown that Gimp is first class graphic editor. Maybe one flaw is missing or hard-to-find documentation for some features (I've learned by accident *so much* I'm quite sure there *should* be more docs!).
But, all things considered: make neat things with Gimp, that's best way to grow the user base. There is no better argument!
![Smile :)](./images/smilies/icon_smile.gif)
@Griatch: There's nothing wrong with Scheme, just remember not to try to work on any other style language on the same day! Saves you from headache.
![Smile :)](./images/smilies/icon_smile.gif)
Scheme has quirks, but they are *powerful* quirks. Then again, Python is fine language too... (Some quirks there too, however...)