Ah, so the issue is fitting the screenshot into a laptop. There might be a few things at play here.
Are you using Layer Masks in Photoshop to accomplish this? If not, select the layer of the laptop in your Layers panel and click on the third icon from the left (a circle inside of a square) at the bottom of the panel. It will create a white box next to your layer preview thumbnail. It should automatically select the white box, but if not, go ahead and click on it. Then make a selection around the screen on the laptop and press delete. Place the screenshot of your site behind the laptop layer and adjust to fit within the dimensions of that selection.
For some reason in my version of Photoshop, it will show a bit of the original background behind the mask. To hide this/make it fully transparent, right click on the mask in your Layer panel and choose "Refine Mask". Then under "Adjust Edge", move the "Shift Edge" slider from "0%" all the way to the left to "-100%". Your View Mode should also be set to "On Layers" (which is hopefully the default, but just in case).
If the problem you're having is that on resize, the screenshot looks "fuzzy" or not as crisp, follow these directions - http://www.missiondata.com/blog/design/372/sharpening-screen... and tweak as necessary.
TL;DR, open/paste the screenshot as two new images, go to Image > Image Size and set the Resample Image (at the bottom) to Bicubic for one and Bicubic Sharper for the other. Put the Bicubic Sharper image layer over the fuzzier Bicubic one and fade the opacity out as necessary. You may need to apply the Sharpen filter to make it a little more crisp too.
Hope that helps!