Where to override images rendering
The function is theme_field() which could be customized by having the right override of the field.tpl.php template in your theme (through template suggestion, see the theme_field() function documentation.

Inside this function/template, the image is rendered by a field formatter. The print render($item) statement print that rendering. The exact used function/template depends on the configured formatter. If your are usign the default formatter, that would be theme_image_formatter().

Note: See the theme() function's documentation for how to override the theme_* functions in your theme or module.