If you need to make a professional looking graphic for your headers or banners it is no secret that Picmonkey and Canva are free, easy, and accessible.
However, maybe you didn’t know about that, yet.
It is easy to sign up for free access to most of the features and use the online editor. However I use the premium, for the extra features. Want to access those? Use my handy affiliate link for Picmonkey, and support me while you do yourself a favor and gain some really useful apps for graphics.Picmonkey
Here is a link for Canva:
Canva is very clear that their graphics are for personal use only. So nothing made on their site should be sold, although I am sure that you can make personal headers for your store or collections on this site.
Picmonkey has no dire warnings, so I am not sure about the use of your graphics, although they will be perfectly fine for anything personal, as well. Personal use is posting on your social media, blogs, etc. as well as making products for yourself, like cards you want to send.
Commercial would include POD products like those sold on Zazzle, etc.
Right now the only things I am recommending these sites for is the personal usage. Making Facebook headers, headers for your store, or profile pics. Collection and category graphics which can all be branded with your particular style.