Take a screenshot of the picture in so that it saves to a file (in windows there are a few different possibilities, Fn+Prt Sc, or Windows+Prt Sc, or Fn+Windows+Prt Sc, it often dipends from device) and open that screenshot on photoshop editor. Take the rectangural selection tool (5th in the left side toolbar, but as you can reorganize your toolbar, that might be different) and select the part of the picture you are interested in and copy (Ctrl+C). Open a new screen (Ctrl+N) and press enter as the window size will be the same as the size of you copy. Past the picture (Ctrl+V). Now you have the picture you want to use and you only need to clean the borders.

Choose the Magic Wand tool (7th in the left side toolbar) and select outside the border. On the upper side toolbar choose Refine Edges.

Photoshop refine edges

adjust Feather and Contract/Expand

Adjust Feather and Contract/expand as in picture or as you need for your choise.

Now choose the Quick Selection tool beneath (8th in the left side toolbar) and if you look at the sympol you see a +, which means you want to add. Pressing Alt it becomes -, that is you take away. With alt pressen and the left mouse pressed you unselect the parts you don't want to add. When the selection is cleaned of the parts that should remain, you press delete button to take away the rest.

Now you have cleaned the borders save the picture as .png and you are done.

God luck and if you have questions, don't esitate to ask