Yesterday we discovered that Squarespace was having trouble the their API. We sent them an email asking about this and they responded with the following.
Thank you for taking the time to email us here. Our developers have this as a known issue and are working on a fix as soon as possible. I've marked this message for follow up and we'll let you know as soon as this is resolved.
We appreciate your patience — Thank you!
So if you are having trouble posting or uploading photos to your Squarespace 5 blog then it will be resolved as soon as Squarespace gets this fixed.