...having the content of those pages just display in the iframe

Ouch. Putting everything into an iframe just to preserve rotation order seems extreme. You can't just randomize the start of the rotation?

Basically you have to let the server know where the rotation is. Eg, you intercept every link click and append a get get var (for example), or regularly submit a ping behind the users back with the current rotation that is saved in the users session, but without some sort of intervention the server has no way of knowing what is happening on the client.

Another creative solution - if you know that each image is display for 10 seconds, you could start the rotation at a position calculated from the current time. Eg, if there are 6 images, you could start at image n where [n] is the modulus of the current seconds in the minute / 10. Ie, everyone who hits the site at 0 seconds gets the first image, at 10 seconds the 2nd image, at 20 seconds the 3rd image etc. When you navigate away, you should theoretically get slotted back into the correct part of the sequence