Personally, if it's just a rounded rectangle, I would try to use box-shadow (if it doesn't have to look identical in older browsers).
You can do a transparent 24-bit PNG (I think it's actually 32 bit, but Photoshop calls it 24 bit), but this can cause problems if you are using a css grid system.
For example, if the rectangle is the exact width of your grid cell, and the shadow goes **outside** of the grid, then you have to fudge your margins to make it fit.
Also, the page will load faster with box-shadow css than if you use a PNG background image.
If I am understanding you correctly.