[hvb2]

I know of an invoicing system that updates the image when it's paid. Seems pretty useful to me.

And yes, that means that an image with an amount is publicly accessible, so what, there's no information about the invoice in there as that's in the text of the email.

[SiempreViernes]

Bet they send a separate mail when you paid though, in which case updating the picture is not much more than a means for them to hide errors.

I subscribed to the daily headlines from a newspaper, they delivered them as a remote picture in the mail. Only it was always the same remote picture each day, just updated. So if you didn't open the mail each day too bad: you snooze you loose, those past headlines are gone.