advice on accidental hiding of text as regards search engine optimisation

Just wanted some advice really. Just been having a look at our website that a guy here has been creating. I know that Google will come down hard on you if you attempt to hide text, as people do, to try and boost their position and ranking on the search engine.

So, I was looking at the code and realised that what looked like text is in fact an image. It is for a navigation button. Code below. What is actually showing is an image and the text is below (unseen). This is accidental, he isn't deliberately trying to hide the text. The text hidden beneath is the same as the text on the graphical button.

My question is, is this bad and will google come down on us for what is, in essence, hiding text? We could go and change them all to text links, no images. Would you say this was the safer thing to do? Just means a few hours extra work to go and change it on all pages.

So here is the html on the page for the button.......

<div class="nav_button">
<a class="button11" href="website/pages/people.html">Our People</a>
</div>

and that class "button11" is defined in the css as......


a.button11{
width:210px;
height:42px;
display:block;
text-indent:-5000px;
background:url('../../website/images/button11.png');
text-decoration:none;
float:left;
display:inline;
}

a:hover.button11{
background-position:-210px 0;
width:210px;
height:42px;
}

Thanks.