Anyone with a computer will be able to use these. You'll get the screenshot from the exact moment of an anime (or webpage) you want! Then you'll be able to use the image for an avatar, wallpaper, or even your profile. There are two methods I know of. Thankfully they're both really easy.
Example of a screenshot from the Idolmaster anime:
Here's the standard way, an alternate way for video screenshots is provided at the bottom if this doesn't suit you for some reason:
1. First, go to the video or webpage you want the screenshot from. For videos, it can be on Youtube (or similar site), or it can be a video on your computer! The quality of your video screenshots will depend on the full-screen visual quality of the video you're watching. So get the best quality video you can! If you're taking a shot of your list or another webpage just make find the page its on.
2. Find the part you want to copy. If its a video- play it, full-screen it, and pause at the part you want to take a screenshot of. If you're taking a screenshot of a webpage or list make sure you're at the exact part you want a copy of and proceed to step 3. You can use the Zoom options under View.
3. Now look at your keyboard and find the Print Screen button on your keyboard. The button might be called PrntScr or something similar.
4. Click that button (while the part you want is up and paused). Nothing will probably appear to happen when you hit the button, but the computer actually saved an image of the screen (kinda like if you hit Copy over an image or other file but not exactly). This is a screen capture or screencap. You'll have to Paste it into a image editor program, so make sure you don't Copy anything new before you Paste the image into the program!
5. Open up an image editing program on your computer. Can be programs like: Microsoft Paint, Photoshop, Paint.net, Gimp, Macromedia Fireworks, JASC animation shop etc. (Microsoft) Paint is best since its fast and easy for this, and most people have it under All Programs and Accessories. If you don't have or can't find any of these or a similar one that you'll need to download one. Most are free and come with trials easily found on Google.
6. Paste the screen capture into the image editor. Usually the option is under File or Edit. You may need to start a new project or right-click around until you can find the option to paste it. If you find an option to paste but the screencap isn't whats being pasted, then maybe you Copied something else after or didn't hit Print Screen while the screen you wanted was up. Just try again. If you got an all black screencap, check below step 7.
7. Save the screencap to your computer! Now just save or export the image to your computer whle its in the image editing program. Usually you have this option through file or by right-clicking the screencap in the image editor!
Did you get a black screen instead of an image?
This usually happens when you try to take a screencap when your video player is in Overlay mode or something. So try to change your video players settings in View to something else besides Overlay mode. Then take your screenshot again from the video player.
Alternate method using media players (a little advanced)
You'll can easily make a screencap by playing your video in one of your computer's media players like VLC player or BS player. Both are free and can be downloaded from using Google. Most video playing programs have an option to save screencaps. I know for sure VLC Media player does: you hit Shift + S at the same time to take a screencap instead of print screen. For BSplayer, you'll simply hit P on your keyboard when you want it to save a screenshot!
In VLC player, under Tools/Preferences/Video Snapshots, you can change the folder (called Directory) where your screencap images go. Change the directory to a place you can actually get them from easily like your desktop. In BSplayer, the directory folder is under Options/Preferences/Frame and Thumbnail Capture/Folder for Captured Images.
Improve quality of your screenshot!
If you're using this method, then you can increase the color, brightness, and visual quality of your video to increase the quality of your final screenshot! Be sure to use the best videos you can find, with bigger screen resolution. If you got a 1080p video, you'll have an amazing screenshot!
Using videos from Youtube and similar flash sites for this method
If you want to use a video from the net in one of these media players to get your screenshot its easy too. You'll just have to download the video from the net, and its easy too nowadays. For Youtube videos, use Firefox and install this add-on: https://addons.mozilla.org/en-US/firefox/addon/download-youtube/
They're going to be in MP4 format and can be played in your media player with the screencap option! Make sure you have codecs to play MP4 (you probably do but Google for one if you can't play them). I believe FFDshow works well for that.