This thread is 5 years old and it’s still an issue. Rather than figure out how to prevent attachments from signatures (versus other attachments), I’m just asking for an option to block all attachments project-wide: Option to disable attachments from email to be added to the Asana task
We’re asking users to send emails to x+PROJECT_ID@mail.asana.com (actually we have a forwarding address that’s friendlier) to raise issues and it ends up creating an Asana task. Unfortunately, every time someone does that, we get upwards of 12 (!) images from their signatures being added to every task.