Same problem here after installing sr3 the background image shrinks when rendering....never encountered it before.  Using Poser 4 renderer is fine...it's only with Firefly - and it doesn't matter what size the Texture Size is set to.  This is a definitely a bug and I hope it's fixed soon 'cause I"m fed up with Poser's quirks after paying good money for the darn software I want it to work properly.