The only method which guarantees layout consistency is to apply a frame style on the images.
But you must stick to a very strict approach: you must not under any circumstances “touch” the iamge with the mouse (resizing or moving) because this creates direct formatting (DF) and frame styles are extremely sensitive to DF, much more than the other style category. DF will override what is defined in the style and, once set, it is next to impossible to get rid of it.
A frame style can define the size of the frame but it is preferable to set the image size before pasting them into Writer. As a general rule, prepare your images so that Writer uses them “as is” (size, crop, pixel density, …). Position can be made relative to a variety of references (in other words, no matter where you paste the image, it can be sent anywhere in the page and react predictably to edits in your text).
One point to carefully look at is the anchor. Avoid To page: it does not do what you think. It is provided as a bridging feature for desktop publishing work (DTP) where pages are the main object to manipulate while Writer is page-oriented (pages don’t matter, they are allocated on demand).
What I can’t see in your screenshot is the relationship between text and images. Usually an image is “attached” to some text and remains in the vicinity of this anchor. Tell us more about this relationship for a more ad-hoc advice.
I have little experience with “PDF images”. I’d rather avoid them because the graphical elements in them will not scale well, contrary to text. PNG and JPEG will be fine; but follow my advice to tune them outside of Writer so that no unintended scaling occurs inside Writer.
A final remark: I assume your document is saved .odt, otherwise there is no “clean” solution because frame styles are specific to Writer.
