[ClemSnide]

Both of you are quite correct, as I also found. I had the bright idea of using the Tutorial frame (which doesn't suck, and looks more or less the way I want, without the extraneous elements of the other frames).

That definition goes into the Backdrop element of the frame:

<Backdrop edgeFile="Interface\TUTORIALFRAME\TUTORIALFRAMEBORDER" tile="true">
	<EdgeSize>
		<AbsValue val="32"/>
	</EdgeSize>
</Backdrop>

(I later use a color fill to handle the internal color, so I don't need a bgFile.) However, I also wanted a badge at the top left. So in the ARTWORK layer went

<Texture file="Interface\AddOns\MyAddOn\UI-MyBadge-TopLeft">
	<Size>
		<AbsDimensions x="128" y="128"/>
	</Size>
	<Anchors>
		<Anchor point="TOPLEFT">
			<Offset>
				<AbsDimension x="-7" y="10"/>
			</Offset>
		</Anchor>
	</Anchors>
</Texture>

This overlaid the graphic on top of the Tutorial frame. I found by experience that the best way to do it was to make the bulk of the badge transparent, so that it didn't place unwanted colors in its rectangle. (I actually believe that one of those dimensions could be 64, which ought to work-- WoWWiki says you can mix values as long as they're both powers of 2.)

Guess what? It works. In fact, it works so well I did my little Chicken Dance of Victory when I saw it. I'm planning a small tutorial on this particular aspect of modding, with the tools and procedures and everything.