PNG transparency isn't supported well (i read somewhere that some browsers doesn't support it - opera i think). I guess you try to make transparent background - try RGBA, new in CSS3 - I think that browser support is good now. RGBA def.:
/* standard color without transparency */
background-color: rgb(0, 0, 0);
/* browsers that support rgba will overwrite the first rule - transparency is set in percents divided by 100: 92% == 0.92 */
background-color: rgba(0, 0, 0, 0.9);
To save a transparent PNG in photoshop, your image first must be edited to delete the background layer (i.e. the area you want to be transparent). Then use the 'File > Save for Web' option, select PNG-24 as the file type and it will save with transparency.