#!/bin/sh
echo -n "Output file prefix: " >/dev/tty
# ^ allows redirecting output to capture the generated html
read pfx
root="/wp-content/uploads/`date +%Y/%m`"
let i=0
for f in "$@" ; do
	let i++
	base="$pfx-$i"
	path="$root/$base"
	# paragraph break for spacing between photos
	printf '\n<a href="%s.jpg"><img src="%s-640px.jpg" alt="%s" title="%s" class="aligncenter" /></a>\n' "$path" "$path" "$base" "$base"
	convert "$f" -auto-orient -geometry 1920x1920 "$base.jpg"
	convert "$f" -auto-orient -geometry 640x640 "$base-640px.jpg"
	# ^ duplicating the dimension applies it nicely to whichever is greater
done

And on the second:

Come to think of it, the first three substitutions there also work for any sort of code import that might contain those pesky &<> characters

I improved on this concept with some cleverness picked up along the way (mostly from MP, if I recall): the greater-than doesn't technically need quoting, while the double-left-paren and proposed square bracket notation for footnote and code snippet preprocessing can be quoted in the same manner. raw2wp:

#!/bin/sh
sed 's/&/\&amp;/g; s/</\&lt;/g; s/((/(\&lpar;/g; s/\[\([0-9]*\)\[/[\1\&lsqb;/g'