Images with text in Message Templates in a multilingual installation
I have found an alternative solution which worked perfectly. 

I put the following line in the template (adding the {ts} tag): 

{ts}&lt;img src="WEBSITE_URL/images/Banner_EN.jpg" /&gt;{/ts} 


Then modified the language file .mo and added a translation for the string: 

msgid "&lt;img src="WEBSITE_URL/images/Banner_EN.jpg" /&gt;" 
msgstr "&lt;img src="WEBSITE_URL/images/Banner_IT.jpg" /&gt;" 


It worked perfectly :)