Wordpress comes with a simple image editor built in. This post gives step by step instructions and guideline using screenshots.
Before you start, a few tips...
If you want to do a lot of fancy edits, add text on it, you're better off using an image editing software. There are some graphic editing programs available online. If you're not very "techy" then the best bet is to get someone who's comfortable with tech to do the image enhancements for you - save your time and frustration!
Click the images below to see full size
Step 1: Upload or Find the Image to Edit
You may have already uploaded the image on a post and found that it's not looking good on the website, and need to edit the image by cropping it or rotating it. Or, a thumbnail is cutting off important parts of the image and you need to resize or crop it to make the thumbnail look good. Some themes require that the image be rectangular to look right, other themes require a square shape.
Step 2: Click the Edit Button
Step 3: Edit the Image
On this page, there are several functions. On the screenshot below, it explains each function. Review it closely. Recommended: play around with an image, get comfortable with using this tool before actually editing real content for the website.
Step 4: Save the Image
This is IMPORTANT - I can't begin to tell you how many times I clicked the wrong button and lost my changes.