Enable images for all emails

Whilst still under preferences, go to Mail Preferences -> HTML messages and select 'Always load images from the internet'
Add your signature and enable HTML messages

Again under preferences, go to Composer Preferences -> Signatures -> Add. I found it easiest to use Insert -> HTML file to import my signature as pasting the HTML into the Add signature window didn't seem to work as expected.

Whilst still under Composer Preferences enable 'Format messages in HTML' to stop Evolution prompting you on if you want to lose formatting every time you reply to a HTML formatted email.

If you want Evolution to automatically add a signature to all new mails go to Mail Accounts under preferences, select the account for which you want a default sig, click 'Edit' and then choose the desired default sig from the drop-down menu under the Identity options.