Photofun is a “ready to use” SEO-friendly PHP montage script which allows you (or your users) to upload jpg, gif or png images, apply effects and then save and/or share the image (with your watermark).
It features 15 standard layers and 12 layers with filters but you can add as many as you want if you have a basic knowledge of PHP.
Mobile friendly, it features an extensive documentation.
Requirements :
PHP5 and GD (bundle)
Mod rewrite enabled (do not upload the script in a subdirectory)