I virtually never got spam email in Outlook on my windows laptop. NEVER. Having switched to a Mac in the latter part of 2015 I find that I now get a MASSIVE amount of spam, at least 6-10 per day that the spam filters don't catch. At first I blamed the spam filter for not catching all these new spam emails. Turns out, I should have blamed the Mac, or rather, Outlook on Mac.
To answer the first questions regarding why that picture still showed up even though that email address isn't in my contacts, I don't know. All I can say is, "it's a Mac thing". The solution, however, is to disable all pictures from downloading unless you click on a button within the preview screen to allow them. I've tested and verified this solution works.
More importantly, not disabling them from the beginning has led to an increase of spam. Why? It turns out that spammers are pretty smart. The picture included in your email isn't a generic picture that anyone can look at. It was crafted and named just for you. What this mean is that a typical picture on the internet of some wrinkly old man might be called Wrinkles.jpg and anyone with the link could view it. The down side is that spammers can't tell who has looked at it. So, they customized the file with a name that is linked, in their database, to your email address and only you (having received the email) can view it. Wrinkles.jpg becomes slsdifsfno23844t2lsnndfgWrinkles.jpg. Honestly, how many people are going to go view a file with that name? Nobody. Except you, because you're the only one that has it showing in in their email. Once their servers log that someone looked at slsdifsfno23844t2lsnndfgWrinkles.jpg they know that the email address that custom named file was sent to is valid. They know someone is home.
The result is that your email address gets added to a long list of other addresses that they have confirmed as valid and that list gets sold to every spammer on the internet. Every piece of spam that comes to you after that, which includes a picture, reconfirms that you're email address is still valid and you're on a virtually never ending Tilt-A-Whirl of spam.
The solution is to turn off all images in Outlook. Once they can no longer confirm that your address is still valid you will begin to fall off the spam lists. They all have different lengths they'll wait to see if you come back, but eventually you'll no longer be on the lists that are sold around the internet. The only spam you'll get will be from groups who rarely if ever update their lists. Oh yeah, and any new lists you unknowingly submit your name to (but that's a different story).