I believe this might cause some troubles if the cloned image brush/tile are later set to a new image.

The only really clean solution I can think of is to clone the tiles and brushes (non recursively) on set and always free them on image destroy.

I believe this might cause some troubles if the cloned image brush/tile are later set to a new image.

Right. And what happens if a tile/brush is set on an image, but the tile/brush is destroyed? Do we consider this a programmer error? And what happens if the same image is set as tile and brush?

The only really clean solution I can think of is to clone the tiles and brushes (non recursively) on set and always free them on image destroy.

Alternatively it should be possible to implement ref-counting for images. That might break some client's expectations, though (e.g. gdImageDestroy() might not actually destroy the image). However, cloning the tiles/brushes when cloning an image might also break some expectations (e.g. clone image → change tile/brush → draw with tile/brush).

Given that libgd is relatively low-level, it might be better to not change its behavior, but to adapt consumers (e.g. PHP).