OK, here's what I know. A 403 error is a server side permissions error - that is we're trying to access something on the server that we aren't supposed to.
Here's what I think I know, everything appears to be OK in terms of what permissions we have to do the things we want to do. Also it appears that if you try to post an image that has been posted before you're less likely to get a 403 error. And smaller images seem to post more reliably than larger ones.
Now for what I don't know - what is going wrong. My suspicion, based upon nothing more than things that have bit us in the butt in the past is that we're posting too many photos and / or that they 're too big. But that's nothing more than a wild-ass guess.
Sorry.
I guess 'ole Sparky, the hamster, was pulling more of his load than we'd thought.
Don