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From everything I've seen trademarks are expensive... and there is no break for not-for-profits.
If you want to protect an image (logo), you can copyright it for free. There are numerous websites with instructions on the different degrees of copyright, but everything is based off of 'first-use', including registration.
For the lowest risk and cheapest approach, here's what I do...
1. Develop a logo (place a copyright on it).
2. Archive and file the logo (to create an official date of creation).
3. Use the logo somewhere; an email is good. That 'use' creates a date time-stamp for first use. If you decide to do this you may send your 'copyrighted' logo to me if you'd like, I'll email it back to you.
4. File or register the logo...
only if someone appears to 'steal' it. You will have first-use documented as well as previous copyright on the logo.
With this approach I only incur the registration mark costs when, and if, there is an infringement.
But since I'm NOT a lawyer, you may wish to check this out with legal council. My recommendation is
not in anyway to be construed as legal advice.