Correct use of text in alt tags

[AutisticCuckoo]

Which would mean... placement of content images isn't a design decision.

The visual placement of the image is a design decision. You can float it or position it absolutely to make it fit into your design. But the information conveyed by the image should be logically placed within the document structure.

Maybe write all content text first while keeping in mind the images one plans to use on the page, and then later work them in?

That is the ideal way to work, in theory. In practice I've found it to be very rare.