I can't understand a leak -- Possibly Instrument fail ?

Question

I'm trying to fix some leaks in an iPhone app, but I'm still fighting w/ one of them. It appears with Instruments, and I can see something like :

ImageIO 128 bytes (Leaked Object -> Malloc)

But I'm releasing nearly all used object (at least I think), could it be a false positive or some memory leak for the ImageIO library ?

I know that looks like overkill, but I'm trying to understand how memory management works, so I'm pretty interested in details.

Thanks for yr help.