Its been so long since I messed with html email because its not allowed
in most groups...but a quick search found this:

Write a Message Using Rich HTML Formatting in Evolution

To write a message using HTML formatting in Evolution:
Hit Ctrl-N to create a new email message in Evolution.
Open the Format menu.
Make sure HTML is selected. If HTML is not yet checked, click on it.
If it is selected, leave the menu without clicking.
If HTML is not yet checked, click on it.
If it is selected, leave the menu without clicking.

Now you can use all the formatting tools in Evolution's formatting bar,
insert images, graphical smileys and links, and much more.

This link http://www.novell.com/coolsolutions/tip/18727.html
is for inserting images in a html email but it might also help you with
other html settings too.