Posterist - Create beautiful photo collages in minutes on your Mac
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
FotoJet - Free Online Graphic Designer | Collage Maker | Photo Editor
GIMP - GIMP is a multiplatform photo manipulation tool.
QwikBanners - Get your display network campaign going in few clicks, generate all sizes in few seconds, no coding or design skills required
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.