Step 6: Unless you drew your icon with pixel art and already tiny you'll need to scale it down to fit on the bottom of your card. Go to the Image menu and select Resize...
-I've found that resizing it to about 30-35 pixels of height usually provides a good-sized icon.
-Save your icon.
Step 7: Switch to your card image in Paint.Net and use the Zoom tool from the Tool palette to zoom in (you'll be working with your tiny icon so this helps you with moving it around).
-After selecting the Zoom tool you will have to actually click on your image to zoom in. 2 or 3 clicks should be enough.
Step 8: Go to the Layers menu option and select Import from file...
-Find the location where you saved your icon and select that to import.
-This imports your icon as a layer on top of the card itself. This allows you to move it around without messing up the card image.
-Your icon will show up with a box around it at the top left of the card image.
Step 9: Drag the icon to the bottom left area of the card and get it positioned.
-The example below shows roughly where the Early Church icon is located on the new set.
Step 10: Save your new card image. By default Paint.Net will want to save it as a .pdn (Paint Dot Net) file because you have multiple layers. If you want to be able to edit the image further (if you decide that you need to move the icon around a little bit more, for example) you can save a copy of it as a .pdn
-In order to have a copy of the image to upload and share on the message board you'll need to save either a PNG or JPEG (JPG) version of the file (you can select either of these from the "Save as type" option).
-When saving as a JPG or PNG it will ask you to flatten the image (because you can't have layers with JPGs and PNGs). Go ahead and do so.
Optional Step: If you're feeling really adventurous you can also add the card number and/or rarity next to the set icon with the Text tool from the Tool palette.
-I don't know what font Cactus uses for the card number/rarity but Paint.Net defaults to Calibri which seems to resemble it.
-I used a font size of 11 for the card image below.
Finished Card: Congratulations, you've now got a card with your very own set icon!