<INPUT TYPE="text" STYLE="background-image:url(http://www.url.gif)">

Ejemplo sin repetir la imagen:

<INPUT TYPE="text" STYLE="background-image:url(http://www.url.gif);background-repeat:no-repeat">

Imagen de fondo unica y centrada:

<INPUT TYPE="text" STYLE="background-image:url(http://www.url.gif);background-repeat:no-repeat;background-position:center">