Some online tools (no software required)

Canva
"Amazingly simple graphic design"
A web-based design tool for creating social media posts and other content.
You can even edit a video.

Pixlr
Don’t have Photoshop? Not a problem. Pixlr is an easy to use and full featured image editor that runs in a web browser

Piktochart
"Make Impressive Infographics"
An online tool for creating infographics.
Here’s a good tutorial for using Pictochart:

Also


And some offline tools - Design software worth considering

If you’re on a Mac computer:

Pixelmator $30
A highly-rated image editing App available from the Mac App store. It bills itself as an image editor, but it’s really a full fledged graphic design tool. It’s easier to use than Photoshop, costs much less, and is useful for any kind of graphic design tasks that don’t require doing vector art.

Affinity $50
More advanced but also very affordable. The Affinity series (Photo, Design, and Publisher) outshine Adobe’s more expensive applications. These are professional level tools that are accessible, intuitive, and well-supported. Photo is basically the equivalent of Photoshop or Pixelmator. Design is a vector art tool like Adobe Illustrator. And the new Publish is meant to compete with Adobe InDesign. You can purchase each application separately.

Adobe Photoshop (also available for PC)
$10/ month (for the Photography Plan with Adobe Creative Cloud)
The standard-bearer in graphic design and photo editing. Powerful tools, but a steep learning curve.

A couple of well-liked, free options that work on both Macs and PCs:

GIMP (free!)
For beginners, GIMP is the perfect alternative to Adobe Photoshop. "Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done."

Inkscape (free!)
An alternative to Adobe Illustrator, Inkscape is a professional tool ideal for vector art lovers and graphic designers.