Código PHP:
<script>
function enableSend(){
if (document.getElementById("check1").checked == true){
document.getElementById("Listo").disabled = false;
}else{
document.getElementById("Listo").disabled = true;
}
}
</script>
Código PHP:
<input type="checkbox" name="check1" value="1" onclick="enableSend()"><br><img src="images/acepto.png">
<br><br>
<input type="image" value="submit" name="Listo" src="images/subir.png" disabled="disabled">