Ungapped
There is an image editing software built into the Ungapped platform that can crop, enlarge / reduce, add stickers and frames, rotate, mirror, and add text to images etc. It cannot, however, manage image inventory or be used for advanced image editing.
Adobe Photoshop
A great image editing tool that allows you to create professional material. The program is not free but can be purchased on a monthly or yearly subscription.
Pixlr.com
A free image editing program that does not require you to install any software onto your computer. www.Pixlr.com/editor
Paint.NET
A free image editing program that you download and install onto your computer. www.getpaint.net
GIMP
A free image editing software you download and install onto your computer. www.gimp.org
Google+
If you have a Gmail account you can edit images for free using your Google+ account.