contrast for images of text

Would images containing text which don't have the proper contrast ratio
fail at single A? I only see 1.4.3 (which is AA) mentioning the issue of
contrast. I would have thought that images of text should fail at single A
since the user agent has no way of overriding the css background and
foreground color for these images?1.4.1 mentions the use of color to convey
information and would seem a likely candidate to fail such an image, yet no
mention is made there regarding images of text.
Thank you for any feedback