Using shadows for text on background images

text outline is not well supported.

lots of shadowed text on an image.

Four shadows text on an image.

5px glowing text on an image.

15px glowing text on an image.

The good news

Using an outline or a glow is an acceptable technique for creating contrast

Techniques for WCAG 2.0

G18: Ensuring that a contrast ratio of at least 4.5:1 exists between text (and images of text) and background behind the text

... if a letter is lighter at the top than it is a the bottom, it may be difficult to maintain the contrast ratio between the letter and the background over the full letter. In this case, the designer might darken the background behind the letter, or add a thin black outline (at least one pixel wide) around the letter in order to keep the contrast ratio between the letter and the background above 4.5:1.

Contrast and Color Accessibility

Understanding WCAG 2 Contrast and Color Requirements on webaim.org examples of glows and outlines

Font size and contrast

Large Text: Large-scale text and images of large-scale text have a contrast ratio of at least 3:1
What is "large text" according to the W3?

For many mainstream body text fonts, 14 and 18 point is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the default size for body text (assuming that the body font is 100%), but authors would need to check this for the particular fonts in use.

The bad news