What you're trying to make is called a "signature". Search for "email signature" and you'll start finding useful tips.

e.g https://support.mozilla.org/en-US/kb/signatures

I'm not sure what you mean by "alias" (it's not a word that Thunderbird uses).

You can set up a signature for each account, and for each identity within an account.

How you actually implement your various signatures will also come down to how ambitious they are and how comfortable you are with hand-writing HTML code. If you know how to write or if you can generate HTML code then you can create it in or paste it directly into each account or identity's Account Settings. Or set each account or identity to make use of a separate HTML file you have created elsewhere. But I should warn you that signatures built in other software don't always play nicely in an email context.

My preferred approach is to use Thunderbird to create a signature, just as if I were writing a new message, but then save it as an HTML file. (File|Save As…). I like to use the Signature Switch add-on to choose between multiple signatures at the point of composing a message. Whilst I have a big grand fancy signature to use in my work context, it's not appropriate to keep using it in a sustained email conversation; the correspondent has had my full signature at least once and doesn't need it repeated with each and every email message, so on a reply and subsequent messages, I'd use a more compact and informal signature.

How to create signature:

• click on 'Write' to open new write message
• Select all formatting - assume nothing has been selected; default settings are never hard coded. So if you want a particulat font, colour, bold or italic etc, you must select it.
• Type signature eg: Regards and name

If at this point you want to add an image:

• 'Insert' > 'Image'
• Select: 'Attach this image to the message'
• click on 'Choose file' button
• locate an image you created that has been saved in a folder on my computer eg: Documents\Signature Folder - not saved in any Thunderbird folder.

• Click on 'Open' - Image should be shown in the small 'Image preview' box.
• Select: 'Don't use alternate text' or type in 'alternate text'

Options to Adjust the dimensions of image to suit

• Click on 'Dimensions' tab
• Select: 'Custom size' and 'Constrain'
• Change width to eg: 200 pixels - Height is auto adjusted

Option to Set image to be a clickable link:

• Click on 'Link' tab
• Enter url: eg: http://www.anje.co.uk

Click on 'OK' to insert the image.

Regardless of whether you have inserted an image or not, now do the following:

• Save file to a folder on computer.
• You will need to select "HTML Files" in the "Save as type" list box. It must be saved as HTML file.
• Close the Write window.

Attach the HTML signture file:

• Tools > Account Settings of mail account or right click on mail account name in Folder pane and select 'Settings'
• Select ; 'Attach the signature from a file instead'
• Click on 'Choose' button
• locate the html signature file and click on 'Open'
• Click on 'OK'

Open a new Write message and you should see the signature with all the formating you selected.

Loads of good info from Zenos and I also agree that Signature Switch addon is useful.