Pictures without destroy callback are allocated on the stack and never
explicitly released.

free(NULL)

...rather than the whole picture resource structure.

...rather than the whole picture resource structure.

These values are already zeroed a few lines above.

gc.p_sys can now be NULL, if the picture has no planes.

The X server supplies a large but nevertheless finite range of XID for
the X client to use. So lets reuse our handles.