Curved Corners, fixed size

I am trying to use CSS to apply a fixed sized graphic over an image on my HTML page. I have done this before but totally forgotten how I did it and can’t find anything on google.

Basically I created a transparent GIF file that have curved corners to match my back ground. I want this to overlay a fixed size image on my page. Everything I tried in my CSS file is making the GIF appear below the image on the live site.

Can anyone give me any suggestions or anything that I have to include in my code.

.floatimage {
float:left;
height:150px;
width:150px;
padding:0px;
margin:0px 10px 10px 0px;
background:#000 url(../layoutimages/imageborder2.gif) no-repeat;
}

Cheers