True text has several advantages over graphical text and should be used whenever possible. True text is easier to read, especially if it is enlarged. The user can more easily customize the appearance of the text to make it more readable (changing color, size, font, etc.). File size is typically smaller for true text and it can be translated into other languages.
WCAG 2.0 Level AA requires that if the same presentation can be accomplished using true text, then you must use true text rather than an image of text. Level AAA requires that text cannot generally be used within images at all.