Is it more efficient to make a half-transparent PNG, or set opacity/alpha-filter on a non-transparent PNG?

The image in question will be reused about 5 times on each page of a website. I can either set the opacity to 0.5 and alpha(opacity) to 50 or I can lower the opacity of the imag开发者_如何学编程e in Photoshop and save it with that setting.

Is there a better practice to follow?

If the image is going to have the same opacity wherever it's used, I don't see why you should need to add a bunch of opacity declarations with the same values everywhere you use it.

Lower the opacity in Photoshop, save it once, and use it everywhere. In case you need to change the image's opacity later, you can simply open the PSD, change it there, save it once more and the change will be reflected automatically without you having to change all the opacity styles again.

Check the 5th answer of linked question.