First, search for an image online.
Once you've found an image you want, click it to select it.
Right click on the image and choose Copy image.

Next, we need to upload our image to imgur or ibb.co so that we can host it online.
For this guide we'll be using ibb.co.

Go to https://imgbb.com/ and press "Ctrl V" on your keyboard to paste the image. Then click Upload.

Once you've uploaded the image, it should give you a "viewer link." This is not the actual image link, but a link to the web page with the image on it. In order to get the skin to work with Redmatch 2, we need a direct image link, which we'll cover in the next section.

Go to whatever viewer link imgur or ibb.co gave you.
Right click on the image, and choose copy image address.
Now we have the direct image link.

Open Redmatch 2, and navigate:
Customize > Skins > Custom
Paste in the image address we just copied and press Create.