int imagecopy ( resource dest_image, resource source_image, int dest_x, int dest_y, int source_x, int source_y, int source_width, int source_height)
int imagecopymerge ( resource dest_image, resource source_image, int dest_x, int dest_y, int source_x, int source_y, int source_width, int source_height, int merge_percentage)
These two functions are both quite similar in that they copy one picture into another. Both of their first eight parameters are identical:
The destination image you're copying to
The source image you're copying from
The X co-ordinate you want to copy to
The Y co-ordinate you want to copy to
The X co-ordinate you want to copy from
The y co-ordinate you want to copy from
The width in pixels of the source image you want to copy
The height in pixels of the source image you want to copy
Parameters three and four allow you to position the source image where you want it on the destination image, and parameters five, six, seven, and eight allow you to define the rectangular area of the source image that you want to copy. Most of the time you will want to leave parameters five and six at 0 (copy from the top-left hand corner of the image), and parameters seven and eight at the width of the source image (the bottom-right corner of it) so that it copies the entire source image.
The way these functions differ is in the last parameter: imagecopy() always overwrites all the pixels in the destination with those of the source, whereas imagecopymerge() merges the destination pixels with the source pixels by the amount specified in the extra parameter: 0 means "keep the source picture fully", 100 means "overwrite with the source picture fully", and 50 means "mix the source and destination pixel colours equally". The imagecopy() function is therefore equivalent to calling imagecopymerge() and passing in 100 as the last parameter.
Take a look at these two example pictures:
Now, to get those two to merge we need a script like this one: