Here you can find out how to create a PNG file with these tools. ・After starting paint.net or GIMP, click on “File” and “New” ...

The background was left transparent as you see with the checkered colors behind the image in GIMP. The .PNG file format allows for the image to have that transparent background in some applications.

After saving as PNG, you can then convert the finished product to JPEG and work with it from there. Read: How to crop, rotate, and change Image size in GIMP.