A collage can be easily generated by using the
Consider the following 4 uploaded images:
The next URL will generate a collage, based on the first image (face_left) overlaid with the other three. All images are cropped to the same sizes (with "face detection" gravity) and differently located within the frame:
Notice the use of
fl_layer_apply for applying transformations to the overlaid image rather than to the whole generated collage.
More information on how to manipulate overlaid images can be read in the following blog post: