WordPress.org

Can I use HTML in the e-mail formatting?

Do you really need to? If so, please request at plugin page, I might add that in the next release.

Is there a way to upload an attachment? Screenshot or something..

Not yet, but if you really need to please write a request at the plugin page. The more requests I get, the more chances are that you'll see it in the next release.

I've setup everything correctly but I'm not getting the e-mails. What's up?

This may be caused by many reasons. The first thing I'd do is check my spam box, perhaps it's gone there. Then see if you're getting other mail from the WordPress blog you've setup by creating a new user and using the email you want to provide support with. Check 'send password to this user' and see if you get the e-mail. If you don't then there's something wrong with your hosting/email provider. Try setting up SPF or contact your provider for more details.

What's the right way to setup the provider logo?

Well you can upload the little logo on your own domain somewhere and then just provide a full URL, though I'd suggest you upload it to the Media Library of your client's blog and catch its direct link from there.