My top recommendation is actually a website: www.picmonkey.com.
a really user-friendly web app that's actually surprisingly powerful.
You can resize images, add filters, adjust exposure...all the important
stuff. Another excellent alternative is Pixlr, which is also available as an extension for Google Drive.
If you're looking for software that you can
download, you've got a choice between phone applications and desktop
applications. My two favorite phone apps are VSCO and Afterlight.
VSCO is subtler and has a nice array of Black and White filters, while
Afterlight has aggressive filters, but includes a nice selection of
unique tools like shape-cropping. VSCO is free (but you pay for
filters), while Afterlight costs a couple dollars. I'll also put a plug
in for the Instagram app, which doesn't export photos well, but its
editing tools do most of what you need and they have a few unique
adjustment tools that I haven't seen elsewhere; I'm a big fan of Lux and
For Windows, there are two free options that handle photo editing, both of which we usually install on new TSS computers. Check your list of installed applications and see if they're there. I'd recommend starting with Paint.NET. It's user-friendly and has all the features that most users will need. If you want the Cadillac of free image editors, though, you can also check out the GNU Image Manipulation Program (or G.I.M.P. for short). The GIMP has a reputation for being a powerful alternative to PhotoShop that, like PhotoShop, has a steep learning curve.
Apple's built-in Photos app is a solid editor that provides good functionality, but some people aren't fans of the simple controls, the limits on features, or the way Photos manages the individual image files in your photo library. If you're willing to step your game up and spend a little money, Pixelmator is the next logical step. It does cost real money ($30), but it's a fraction of what you'd pay for Photoshop and it gets absolutely rave reviews.