You don't want a Custom Object and you should delete that object pronto.
Tom, Nicho, et al. are talking about a custom Person/Lead String field. Not a CO, that's way beyond this task.
In fairness, though, part of the exchange above is off-topic and understandably confused you.
After you create a String field (in Admin » Field Management) it's available as {{lead.Whatever Field Name}} in emails.
You would set that {{lead.Whatever Field Name}} as the src of an <img> tag. Not the href of an <a> tag -- that would make it the destination of a link click, when what you want is to pull the image into the email.
It's true that if you include the protocol inside a token value (as opposed to outside) and then use the token as an href, the link won't be tracked. But that's different matter, really.
Adchoices