To take a full screenshot and open in Photoshop:

1. Resize the webpage so it is the size you want
2. Press Ctrl/Alt/PrtScn
3. In Photoshop, File/New/Ok - do not change anything as Photoshop will default to Clipboard and so the new template will be the exact size of your screenshot
4. Ctrl/V to paste screenshot into template.
5. Resize as required.

To take a screenshot of a right-click or pop-up menu (this disappears when you use the above method):

1. Run snipping tool ( If screen appears grayed out, press ESC, if not continue step 2.)
2. Click to open the menu you require
3. Hold CTRL and press print Screen. Screen will appear grayed out, ready to capture.
4. Use snipping tool as required and save.
5. Resize in Photoshop as required.

Adobe 

For screenshots of Adobe products, please refer to the Adobe Products - Screenshots and Screencapture page.

 Microsoft Licence requirements:

You may not use screenshots of Microsoft product boot-up screens, opening screens, "splash screens," or screens from beta release products or other products that have not been commercially released. You may use other screenshots in advertising, in documentation (including educational brochures), in tutorial books, in videos, or on websites, provided that, in addition to the requirements above, you:

• Do not alter the screenshot except to resize it.
• Do not use portions of screenshots.
• Do not include screenshots in your product user interface.
• Do not use screenshots that contain third-party content.
• Do not use screenshots that contain an image of an identifiable individual.
• Any mention of Microsoft products in the text should have the appropriate trademark - see links:
  https://www.microsoft.com/en-us/legal/intellectualproperty/trademarks/usage/general.aspx
  https://www.microsoft.com/en-us/legal/intellectualproperty/trademarks/en-us.aspx#096c2b52-4010-4e38-9c79-59a828f1500e

Google Chrome permissions [extract]:

You don't need our permission when you want to use an unaltered screenshot of our homepage (Google.com) or the search results page for instructional or illustrative purposes. This applies for print (book, magazine, journal, newspaper) or digital (web page, DVD, CD) formats. Please see below for exceptions to this rule for some of our other products.

https://www.google.com.au/permissions/using-product-graphics.html 

We ask that you put the following tagline beneath any image featuring a Google product screenshot or data:

• Google and the Google logo are registered trademarks of Google Inc., used with permission.

Examples of copyright attributions for screenshots from Microsoft and Adobe.

**Note that Microsoft and Adobe require that the entire screenshot is included (not portions) and they may not be altered in any way.

Microsoft <product name> screenshot used with permission from Microsoft http://www.microsoft.com/en-us/legal/intellectualproperty/permissions.

e.g. Microsoft Word screenshot used with permission from Microsoft.

Adobe <product name> screenshot reprinted with permission from Adobe Systems Incorporated http://www.adobe.com/au/legal/permissions/images-user-guides.html.

e.g. Adobe Dreamweaver screenshot reprinted with permission from Adobe Systems Incorporated.

Screenshots of copyrighted software

Use the Wikipedia guidelines for non-free software screenshots to guide you: https://en.wikipedia.org/wiki/Wikipedia:Non-free_use_rationale_guideline

Cover up any product names, e.g.

Attribution used for this example:  Screenshot of licence information for non-free software used for educational purposes only.