ShoeBox - Free unlimited photo storage, so you can rediscover your photos anywhere. Available for iPhone, iPad, Android, Mac and PC.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
TexturePacker - Texture Packer is hands down the best sprite sheet generation tool on the market.
GIMP - GIMP is a multiplatform photo manipulation tool.
Free Texture Packer - Free Texture Packer is an open source tool that allows you to pack multiple images into one atlas for you games or sites.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.