Unrelated to OS. I know an Apple user who has had same thing happen.
In which case I'd suggest its a nefarious exploitation of a technique employed by email marketing companies.
Image is hosted remotely, you open email, image loads, they correlate on their backend and hey-presto ... they know you've opened the email and they obviously know which gmail address they sent it to. Marketing companies use this technique to track readership statistics, the dark side... well, you've just found that one out !
On a Apple, if you're running Apple Mail, if unsure about the intentions of a message the answer is to mark the email as spam (thumbs down)
BEFORE opening it.... then you can open it, because Apple Mail doesn't open remote resources on any messages that are marked as spam. You can then read the text of the message and get a feel for it and decide whether you want to load resources or open attachments, or whether you want to make enquiries with the sender first.
But the old rule still applies... if you're not expecting emails with attachments, even from people you know, then treat with care !