Blank spaces in epub due to anchored images

First, I am not sure the images or anchors are causing the white spaces you note. There are text formatting issues that would do it, as well.

Second, a lot depends on the reader you are using; different ones (especially ones with different code 'DNA') will display things differently.

Third, there is no good provision in EPUB for text to flow dynamically around placed images. EPUB is a linear document format and text and images will appear where they lie in the flow; if an image takes half a page, there may not be enough space for more text on that display page and there is little you can do about it. You can use (CSS) float, but it's buried in the individual image settings, or via attached CSS, and it may not solve all text flow problems well.

But mostly... depending on the layers-within-layers of spot element definitions and overrides and so forth to either tell you much about the export or to try and adjust faults is almost useless. By default and without careful management of the InDesign document, those on-the-fly HTML containers and CSS defintions will change on every export.

Since you're obviously comfortable with it, I would get into CSS styling for your book elements, and start by assigning a named Object Style to the images so that you can apply direct, consistent control to them on export.

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Howdy, good morning (for me anyway)

There's a thread here with a similar query 

https://community.adobe.com/t5/indesign-discussions/how-to-anchor-existing-objects-goal-reflowable-epub-w-placed-art/td-p/10175600

Let us know if it helps 🙂