Text within images can be difficult to read, especially for some people because:
images don't scale when text size is increased by the user;
magnified images can appear highly pixilated
(or fuzzy):
particularly for small (and anti-aliased) text.
Using text properly reduces file size, permits scaling and easy updates, and reduces potential for accessibility errors (such as missed alt text).
If possible, always use text rather than images of text. Exceptions:
logos;
important special text effects;
important brand communications;
illustrations (such as in this document.)
Avoid putting essential text within an image.
Avoid putting large paragraphs of text within an image.
Avoid using images of text for important navigation (menus, etc.)
Tip: Often the same visual effects can be produced using CSS to style text and background images.
Tip: Where text as images is used, check the clarity by magnifying the image. Sometimes minor adjustments to size or position can improve the fuzziness caused by anti-aliasing.