You've already received some pretty good info and if you've settled on a solution that's fine but here are a couple other alternatives.
First, the "help" section of Win8 really is quite good (unlike previous versions of Win). Next time try going there first. Bring up the "Charms" bar and click on the "search" icon (magnifying glass at the top or left). Type in "screenshot" and you'll get a number of alternatives. In addition to the search results you'll also find other helpful links including "files" which is a big help in locating relevant Win files when you need them. (to place a shortcut to the "snipping" tool on your desktop, for example).
While "snip" or the keyboard shortcut to print screen will both work, Snipping Tool is much more flexible so of the two I'd recommend Snip, especially for grabbing just a portion of the screen (like you want to do here).
Despite the fact that snip is MUCH better and more flexible, there are other (free) alternatives out there that offer many additional features, many of which you might find useful.
My favorite is irFanView. Despite being very feature rich, it's really pretty easy to get "up to speed" using the basics (grab, save as, crop, change file type, etc). As you get more comfortable and explore the other features you'll find there isn't much you can't do right inside irFan (change/reduce file size, add effects, highlights and "paint" features, videos, even create self running slideshows).
You can "grab" the entire desktop, or just the active pane, and once the grab is inside irFan you can cut/paste just a portion of the "grab" (the ad you want for example), and save it as a separate file (keeping the original grab in case you want to cut another portion).
The download is safe and the author updates frequently with added features and "bug fixes". I've been using it for years, tried many others, but always come back to irFan as my "go to".
If you decide to download/install, add (pin) irFanView to your taskbar.
To grab a screenshot,
1) hit the "prt sc" button on your keyboard (nothing will happen that you can see, but the screenshot has been copied to your clipboard.
2) click on the irFan icon on your taskbar to open
3) select "edit" then "paste"
4) you can now "save as" or continue on to the edit the grab, to edit;
5) left click and hold (starting from top, left corner of the area to want to save), move cursor to bottom right corner of area to save and release left mouse button (box shows area to be saved)
6) Select the "cut selection" icon on toolbar (scissors) or "edit" then "cut selection" from the drop down menus
7) Select "edit" then "delete (clear display)" from dropdown
8) Select "edit" then "paste" from dropdown
9 If you are satisfied with the results, "file" then "save as"
It takes way longer to type out than to do in real life once you get the hang of it and it really is worth the effort once you see what all you can now do with the file.
After you've "saved as", click on the "Images" drop down and just play around.
Look too in the "Edit" section for the "show paint dialogue" button (if it's not there or is grayed out, you might have to download/install the "add ons" from the irFanView website (not sure it's been a long time since I installed).
Have fun, it really is an amazing program, every day you use it you'll find something new.
Here are some useful links to get you started. www.youtube.com/watch?v=iLmWWeyhILM www.irfanview.com/ fileforum.betanews.com/detail/IrfanView/96
All Cows have four legs,
Rover has four legs,
Therefore - Rover is a Cow.