Tom Walsh, what makes you think that these are always Internet service providers that block images? What about email clients? 80% of email messages have image blocking by default. Relying on users to make an effort or knowledge to change these settings is a bad idea.
Using some services, such as Mailchimp, each time you send, you get a new email address, so you cannot add it as a safe sender. Techniques such as those mentioned are a great way to get a list of email addresses that has already been turned on twice to get a semi-graphic version of your email, to encourage them to upload images.
This top link works. I used JPG, I'm not sure which format it accepts. Just to let you know, this makes the file more believable than regular JPG. Think about whether to use it for large images, but for email, I see that it is a worthy use. I noticed that sometimes when you reply, signatures break ... sooooo .... know that you can get a load of <tr><td>A</td></tr>
Final warning ... if your email signature is broken and ends in code form .... remember that the default for the top link is "ass", therefore ... some customers may find this offensive. Perhaps change it to your company name. And remember that creating a link for this table image will (a) take some time and (b) pretty much inflate this code a second time.
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