I have a web app, and I want to take a good screenshot of it, for its landing page. This is an example. Anyone know how to do it, on on a macbook air Here is an example
First of all, use a computer app instead of a web app to take screenshot. Here are some good screenshot application for Mac
Next you need to set your screen resolution to the highest possible resolution and it must take the screenshot in a pretty good resolution. Also check if there's some settings for the screenshot application regarding to the resolution of images.
Take the screenshot without any app
On Mac you can take screenshots without any additional app. See this to know how http://osxdaily.com/2010/05/13/print-screen-mac/
Then you can use the image on the clipboard to edit in an image editing software. Read this for a trick to get high resolution screenshots in photoshop http://www.turbophoto.com/Photoshop-Tricks/screenshot-photoshop-trick/index.htm