Gallery problem

I have 4 small galleries on a Twenty Twelve site I am building. All work fine excepting for the final one which has 5 images but keeps on including my header image as a 6th item when you browse the gallery using previous and next. None of the other galleries do this. Any suggestions what could be happening here? The header is definitely not part of the gallery when I look behind the scenes. I have Nextgen installed but I am not using the platform as such. I don’t know whether the gallery I’m using is part of Twenty Twelve or whether this bit of software is being picked up from Nextgen (if it’s possible.) Nextgen is very different from this – but I like this whatever it is.