If so and they don't work, you might have to enable IMAP and or non-secure apps in the Offlice 365 settings (via the webmail or something). You might also have to make sure that any 2-step verification login settings are off.
Another way to do it is to use https://software.hixie.ch/utilities/cgi/data/data to generate a data URI for the image. Then you can use that in src=. That way the image is embedded in the HTML markup in the email. The downside to this is that not all clients support loading data URIs.
First, realize that in Unread, messages are not marked as read until you actually mark them as read - the colors only indicate seen versus unseen. It would be possible for you to set Unread to show everything - not seen (blue), seen but not marked as read (bold black) and read (regular black) - but if you're not seeing all three styles in the list then that is not your problem.
To actually mark a message as read, you can click the button above the text, or press K, or right-click a message in the list and select Mark > Mark as Read (there are other options as well, which you'll see on the menu). I'll presume this was probably the issue and not discuss the other possibility unless you tell me that you are actually seeing messages you marked as read in the Unread view...
If you find the solution to export Opera Mail MBOX or MBS file to Outlook compatible file format, so I would like to suggest you DataHelp Opera Mail to Outlook converter, with the help of this software you can easily convert MBOX, MBS file of Opera Mail email client into Outlook PST format.