Hi Matthew! My name is Shrev, and I’m the Fandom Wiki Manager for Club Penguin Rewritten Wiki. I am here to help the community and be a liaison to full-time Fandom staff. If you ever have a question or issue relating to the wiki, editing, styling, infoboxes, templates, etc., please feel free to contact me on my message wall, or on Discord at Shrev64#0089.
Assuming you're having trouble about the actual uploading part, if you hven't already try using this link for your file. Also if the file is too big then it may not be accepted. https://club-penguin-rewritten.fandom.com/wiki/Special:Upload If you simple want to change your avatar to a custom image then go to your own User Page and under your current avatar you can click "Edit avatar" and you can upload your image from there.
As it is explained I do personally think he's talking about his playercard. I'm not sure about this as it is confusing tbh. ^^
But, if you want a custom penguin with no background, then you need to render it with the approrpiate swf. if you have no idea how to render, then I can render it for you.