Thank you everyone!
As suggested I asked: “Can I confirm the master bedroom is a choice? From the photos I thought it would be since it is the main photo. I prefer a larger bed and normally would not apply for a sit with a single bed only.”
The HO replied: “I typically have folks stay in the guest room but I will put fresh linens in the primary so you can use it”
Perfect outcome.
If you feel comfortable now or after the sit, maybe suggest that they don’t post the main room photo, because it’s misleading to post something sitters won’t get to use.
@Kalina that’s great you spoke up for yourself & got the right outcome. But strange that the HO posted a picture of the master room but does not usually give it to sitters! That’s very misleading. Her response to you sounds a little superior too… I hope it will be a good sit for you.
If the sit doesn’t take place due to misrepresentation, no reviews can be left.
Sorry, i should have read the entire thread before posting.
Agree with @Lokstar on that @Kalina - we’d respond with something like “Great news, thank you. You’re actually really lucky it hasn’t been flagged by a sitter before as THS are super strict on photos being accurate images for sleeping arrangements. You might want to change it for future sits.” Which I know is slightly flippant but, it resets the power balance & puts the onus on THS as setting the rules and you simply being reasonable on your expectations. Hope it turns out to be a good sit. #mutualappreciationsociety
I’d be wary of writing such a strong response @Cuttlefish -even if its justified! Just thinking of the review…
Or maybe just say it AFTER getting the (hopefully 5*) review…
#peoplecanbefickle!!! 
It’s the rebel in me @Lokstar but yes, that may well be wise #jamesdeanalltheway
Kalina, yes we have done a repeat sit where the sitters sleep on a comfortable sofa bed in the spare room. The listing was older so didn’t have photos of all the rooms so THS said they had to add a photo of the sitters bedroom and they added one of their master bedroom instead 
No mention in listing of where sitters sleep so most people of course would presume it is the bedroom shown.