Canva: http://canva.com This is a great web-resident program that reminds me of Microsoft Publisher. It's got a ton of templates and a nice selection of free stock photos and formats. Included is a large library of stock photos priced at $1. Great for flyers, Facebook ads, website graphics and freelance projects. REALLY easy to use.
Gimp: http://gimp.org/ Pretty much Photoshop but free. Mac and Windows. It's been around a long time and is very stable and well-known.
Pixlr: https://pixlr.com/ For your cell phone, iOs or Android. Wide range of effects and capabilities. Primarily a photo editor. Removes red eyes, crops, all that.
Acorn: http://www.flyingmeat.com/acorn/ Another phone app, but more like Photoshop, with layers.
Paint.net: http://getpaint.net/ A version of the Paint program originally shipped with Windows and still Windows-only. Many of the capabilities of Photoshop.
Sumopaint: http://www.sumopaint.com/ Browser or iPad, Photoshop-styled application.
BONUS: Great FREEEEEE screen capture (still or video) at http://techsmith.com -- get the JING free one. Camtasia is great -- same place -- if you need longer or more complex videos.
For more on many of these programs as well as low cost programs, check out the Blog Post at http://www.creativebloq.com/photoshop/alternatives-1131641?page=1
There's also a great collection of free or low-cost alternatives listed here: http://www.theguardian.com/technology/2015/feb/19/25-alternatives-to-photoshop-image-editing
And for 501(c)3 groups, you can get many very low cost deals on Adobe programs if you register with http://techsmith.org . You can check out the Adobe offer at http://www.techsoup.org/adobe Note that if you get the Creative Cloud, you can install it on TWO computers for the same price.