Notifications - Pictures not visible from Mobile or Gmail issue?

Summary:

We would like to know why in our notifications, pictures are correctly seen from Outlook on PC but not from Mobile, and why on Gmail they do not.

Content (please ensure you mask any confidential information):

Hi,

We have an issue with a custom notification. This notification contains a PDF in attachment, so the body of the email is contained into the bursting of the report, PARAMETER5. In the body, written in HTML, we inserted a image insert into the content server with Security Group set on "Public". The image is correctly seen from our outlook from PC, while from mobile, it is not correctly seen. Furthermore, from GMAIL is not visible at all, neither from PC, nor from mobile.