For example SasaMisa who was behind some of the Madoc and Arabatos creations in Feb 2012.
These folk fly under the radar a lot of the time, but we should keep in mind that they aren't just purse-strings (like some anonymous subscriber to a monthly subscription, for example) but also idea-makers.
IMO this makes them artists as well, because they are part of the brainstorming process that goes into creating artwork too, which is why I would also like to credit them where possible.
Sometimes an artist might say in the pic itself (or in notes) who they did it for (unless it's desired to be anonymous)
Other times it might be subtler like the commissioner will upload a copy to their own account and say the artist who drew it.
I'm thinking to help organize them we could have a "commissioners" group (which is what I put SasaMisa in, and I'd like to add more) for historical purposes.
This seems a bit more specific than funder/philanthropist since those terms could also describe people who just give non-specific donations to artists (or subscribe) but without actually providing creative input on particular works.
The idea, much like when multiple artists work in tandem to create an image, is also to recognize the intellectual input from other people whom an artist acknowledges as contributing to a creation.