I haven't seen an official guide for this, but I've found that creating accounts and setting up avatars for your sender email on the major email services (Yahoo, MSN, Google, AOL) is the best way to get your sender logo/image to show in the inbox.
So far Google and Yahoo seem to the be easiest and show the soonest.
I've used a company called FreshAddress. (don't have any affiliation so I won't link to them).
It's not super cheap, but we got back a lot of great data, including fixes for typos and reasons for suggested removals.
We've been using OFT files for a few months now and handing them off to be sent to our dealers.
Our dealers are sent through Outlook groups (ugh) . So I've found that OFT files at least are a better option than someone copying the contents of a URL into an outlook send window and making a mess.
It's not idea, but in our situation it works about as good as you could hope considering the limitations and drawbacks of our current process.