In short:
For details: Ryan Armstrong's Blog - Testing Zabbix actions
By default Zabbix sends plaintext e-mails and has no builtin options to send HTML mail.
See: ZBXNEXT-517 - Mail text as plain/HTML, only on demand
To be able te send HTML e-mail an external script is needed.
Several options exist:
Source: Zabbix Forums - No Background images MAP - zabbix 1.8.2