Patrick – I think that in some cases a document will “make sense” with or without styles (depending on the context, of course) even if some of the content is stashed away in background images. In the case of the transaction/error message, it may “make sense” but I think it just makes more sense to put the image with alt directly in the content.

But, yeah, I suppose I could have written “make sure it makes sense without styles” but that wouldn’t have been much of an article now would it? :)