If you're looking for a tool to merge multiple images into one composite image, Adobe Photoshop is the industry standard with a lot of compositing power. It's geared for pros and serious enthusiasts, with a wealth of controls for text, color and imagery. Adobe Photoshop is available as a free trial version and includes the latest AI tools, so it's a good option for creating high-end imagery.
Another good option is Fotor, an all-purpose online photo editor with a clean interface. It's got batch photo editing, background removal and object removal, which makes it a good option for creating collages. Fotor works on Windows, Mac, iOS and Android, so you can use it on whatever device you have. The free version lets you export without a watermark, but you can upgrade to Fotor Pro for more features.
If you prefer an online-only option, Pixlr has a collection of photo editing tools, including Pixlr Editor for heavy-duty editing and Pixlr Express for quick touchups. Pixlr also has a Batch Editor for editing groups of images and a photo collage maker. Pixlr's collection of AI-powered tools and cross-platform support make it a good option for creating and editing images quickly.
Last, Clipdrop is another AI-powered tool that automates some of the more complicated image editing tasks. It can remove backgrounds, upscale images and generate text-to-image creations, among other things. Clipdrop is easy to use and good for pros and amateurs, and it offers a free version with lower image resolution and a Pro version for higher-resolution images.