Here's part of a thread on the OC forum that appears to fix the issue.
I've had the same problem with my image logo not showing up in order confirmation emails when viewed on Thunderbird, Seamonkey, and my iPad - they seem to show up okay on my Webmail, though.
I don't know if this will help you, but this is what worked for me.
Open catalog\model\checkout\order.php
Line 233 -
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$template->data['logo'] = 'cid:' . md5(basename($this->config->get('config_logo')));
Add '.jpg' to the end, as follows -
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$template->data['logo'] = 'cid:' . md5(basename($this->config->get('config_logo'))) . '.jpg';
Do the same on line 432 -
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$mail->addAttachment(DIR_IMAGE . $this->config->get('config_logo'), md5(basename($this->config->get('config_logo'))));
Change it to -
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$mail->addAttachment(DIR_IMAGE . $this->config->get('config_logo'), md5(basename($this->config->get('config_logo'))) . '.jpg');
I didn't think it would be as simple as that but now the logo shows up just fine in Seamonkey and my iPad. I'm no expert (I'm a complete OC newb), but I imagine this'll only work if your logo is jpg. If it's gif or png you'll probably want to change the jpg to whatever extension you're using.
I only figured it out because I compared the OC admin newsletter email with the order confirmation email and noticed that the ".jpg" was missing from the end of the "cid" number on the order confirmation email. Hard-coding it in like I have probably isn't the correct way to fix it, but it works so I'm a happy bunny.
I hope this works for you.