Web Applications

My top recommendation is actually a website: www.picmonkey.com.

It's 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.

Mobile Applications

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 Structure adjustments.

Windows Applications

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 Applications

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.