Office 365 - Outlook - Add Pictures or Attach Files
Information and a link to Microsoft Support about how to add pictures and attach files within Outlook for the Web.
You can attach photos and documents from your computer or OneDrive to email messages and calendar events. Attachments always appear at the top of the email.
You can also insert pictures anywhere in your messages, so that recipients see the image where you want, and as soon as they open the email. They don't need to be opened to be viewed.
See the link from Microsoft Support below about how to use this attachment feature within Outlook for the web:
*Need more help with Outlook? Then check out the Microsoft Support website for Outlook help & learning.