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I generally ask myself a question to determine if an image is presentational or not. “Will the image’s [potential] alt attribute add new information to the document?” If the answer is yes, then it’s content, otherwise, it’s just decoration. In the above error message example, the question is worded something like: “What information does ‘Your postal/zip code was not in the correct format.’ not convey that the icon does communicate?” The answer is none. Nothing new is communicated by the icon, and so the icon—just like the color—is presentational.
I keep thinking about the adage, “What part of ‘no’ do you not understand?” You may decorate the word all you want, but the message is already communicated as concisely as it can be.
The real estate example however, that’s another story. That image is conveying new information, and it belongs in the mark-up, not in the style sheet.