I have asked the Microsoft.Public.PowerPoint list several times to answer this question. I have asked my MVP lead several times. I have received no response. Apparently, he has received no response. Yes, I am an MVP in the US, in Redmond and within 20 minutes of the campus.
This is the Picture Formatting dialog I am attempting to customize. I want to have a way to define a custom picture format and apply it at the click of an icon in the ribbon.
In Word, the style ribbon is fully customizable:
How can this be done in PPT?
I know there is some way to change the Picture Quick Style gallery, but it was very complicated. You can't remove anything from the gallery as far as I know, and to add stuff you need to know the developer name for that tab, group, and the gallery (TabAddIns was the name for the Add-ins tab when you edit the CustomUI.xml file). You could instead create a button not in the gallery that could do that, but I only know how to format pictures programmatically in Word.
All in all, way to hard for me yet.
Also, you are able to add a Style task pane in PowerPoint, but that's even more complicated than editing the gallery (if you want it to actually do something when you click on it).
I hope you find I better answer 'cause this is all I can give.