Thanks for looking at this.
We are adding sitemaps "slowly" - ie 10 million at a time. eg: http://bit.ly/RDHmyo
This stemap was submitted about 14 days ago and has had about 2 million pages indexed but only 5,000 images indexed.
We will add another this week.
I think you may well be right about crawl budget. I have ensured that the bots are almost exclusively focused on these pages (well over 90% of crawls are of these pages). I am assuming that the only way t increase budget is to increase authority. Is that right?