Why does the checkbox stay checked when reloading the page?

I'm reloading a web page that has the following code:

<label for="showimage">Show Image</label>
<input id="showimage" name="showimage" type="checkbox" value="1" />

Even though the HTML stays sent to the browser is the same for each reload of the page, the checkbox always takes on the checked value when a reload was performed. In other words, if the user checks the checkbox and reloads, the checkbox is still checked.

Is there some caching going on here?

Edit: I tried Gordon Bell's solution below and find that this is still happening even after removing the value="1". Anything else I might be missing?

<label for="showimage">Show Image</label> 
<input id="showimage" name="showimage" type="checkbox" />