Images in email signature display as red x in the recipient's inbox

We have been experiencing issues with images on the email signature in Netsuite. Whenever an email goes out, the recipients see a red x instead of the image. The HTML signature references images in the file cabinet (file available without login checked) but the recipient can only see a red x.
Netsuite support advised that as this is a setting in recipient's outlook (Automatically download images setting), not much can be done here.

Anyone else experiencing this problem and any possible workarounds or solutions?

Thanks in advance.