I don't think you're missing anything basic. I think the thought that you are is a slight block. When a spread has mirror margins, and there are images involved which are not anchored, doing something to the layout which causes recto and verso pages to flip is a major hassle. As @Peter Kahrel mentions, only anchored objects have relative-to-spine positioning, and anchored objects that involve text wrap don't really work well with flowing text (mainly because if the object doesn't fit on the page, it is pushed onto the next page, and takes all the following text along with it, which means an ugle white gap at the bottom of the previous page). So my answer is to leave the fine tuning of pictures (bleed, etc.) only once you're fairly confident that things aren't going to move from one side of a spread to another. And if you do ever need to add a page somewhere in the middle, add a spread, not a page, to avoid things moving, and see if you can fill it up somehow. Another tip: If the book is divided into chapters, it makes it much easier to handle if the design specifies that each chapter opening starts recto. That way, at worst, if you need to add a single page, the side-flipping will only affect pages until the end of the chapter.
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