Hello budy,
Well if you have done the image corectly, i mean you save it correctly, then the problem isn't in the image itself. Now try to load the image in a blank Html page without css to see if the problem is in the style! If the transparent work then try to fix the css in your website. Try to find the rule and fix it!
If you have more questions about this, feel free to ask! :)
O...don't forget to use always RGB colors! Now...if you want turn the style of the div in the image to ...
background: rgb(0, 0, 0);
background: rgba(0, 0, 0, 0.5);
Otherwise just use an image with the opacity set on there. That would cover all browsers.