I'd have no problems paying an annual fee, however I think you will find members not all that excited about paying anything, here. I have no problem and recommend putting up a permanent Donation button, that just makes sense.
I have tried to think of other ways for the Guild to make money.
One idea, (I was waiting to see the results during the December challenge - to see how many people are actually interested in the idea,) I would be willing to offer a printing service, where maps created for monthly challenges or other specifically CG related maps are available for print from Gamer-Printshop.com and that $1 or $2 of the proceeds for each map print will be paid to the Guild each month. This could include both large format and multipage color PDF printing.
But then I didn't want to overly insert myself into Arcana's business - I wanted to see how the December challenge went, before I made this suggestion, but since the question has been officially "aired" - I'm offering.
I have no idea, how many would order, but knowing that ordering a map helps support the guild, people would be interested to a degree.
I still think the Guild needs to create sellable publications for fantasy mapping. Say, gather all the tutorials together, create fill-in tutorials in any areas we haven't covered and create say, the Fantasy Cartographers Guide to GIMP - make it available as a PDF book with 100% of proceeds going to the guild. Include all the best GIMP created maps and make this a coffee table book!
Perhaps contact some graphics application developer and try to create a packaged - buy this graphics app and receive a collection of tutorials and/or map objects ready for use (gather all pertinent map objects from the site...)
Just more ideas, for you Arcana.