@Angela_L So just my $.02 as someone who does this kind of product development for a living. We’ve struggled with similar issues on past projects - trying to get existing users to comply with new standards - and if you were my client, here are some suggestions for solutions that I’d make in order of least intrusive to most intrusive
1 - If the HO doesn’t have a minimum number of images, create a popup when they try to schedule new dates that asks them to add additional images. It’s a quick query to implement to display this message.
2 - same as #1, but make it a requirement so they cannot add new sit dates until they’ve added the requisite number of images. A bit more hard line but it’ll catch the people who are just closing the popup without actually reading it. A more mid-level approach between 1 and 2 would be for them to be able to ignore the popup in #1 x times before it then becomes a requirement.
3 - add functionality to categorize uploaded images with the suggested types. So when they upload an image, they would then categorize it with a preset list like Pet Pic, Living Room, Kitchen, Outside, Sitter Bedroom, Sitter Bathroom, etc. This might prompt them to upload more on-target images. You could also have a popup on profile save that prompts them to add any of the missing image categories. This would be the solution I would push for with my client as I’ve found that if you implement something like this that is more directed within the UI, people are more likely to do it vs just a block of text with recommendations that people will skim but potentially ignore/forget.
4 - Similar to 3 except make uploading the category specific images a requirement for going public with a listing. IMO this is too hard line and will alienate people who would prefer to do a video tour rather than posting images.
Real talk: most people aren’t going to read blog posts and emails. The more guidance you can provide directly in the UX/UI, the better the compliance will be. And it’ll save your staff time and effort.
I would also suggest adding a flagging option directly on listings. A lot of the posts in the forum contain suggestions to PM listings to your team for review. More real talk: most people aren’t going to realize that’s an option and of the people that do, many likely won’t be bothered, they’ll just move on to the next listing because the level of effort is more than many will want to deal with while just skimming listings.
Adding a flagging function directly on the listing and including common reporting options (no internal pictures, goes against TOS, etc.) and a free form “other” reason will make it much more likely that inappropriate/problematic listings are reported and it will also save your team time and effort as it will standardize the information you receive. You could even implement a “x flags and the listing is hidden until reviewed” functionality for the case of extreme problems.
I realize that your resources are limited, as they are with all businesses, so optimizing some of these processes might cost a little upfront for the dev time, but in the long run, the savings on time and effort with the CS team would likely be significant and there will be fewer problems over time as people will be complying from the start.
Anyway, hope this isn’t overstepping. This is just what I do and I tend to geek out on UX/UI and process optimization. 
@Ben-ProductManager tagging you at Angela’s request.