The artwork of your podcast will always be your profile picture on SoundCloud. 

Podcast Artwork Requirement

If you want to submit your RSS feed to Apple Podcasts, you must make sure that your profile image’s artwork is at least 1400x1400 pixels and no larger than 3000x3000 pixels. 

If your artwork is not between these image requirements, you will receive an error from Apple Podcasts that says “Podcast cover art must be at least 1400 X 1400 pixel JPG or PNG, in RGB color space, and hosted on a server that allows HTTP head requests”.

How can I find the pixel dimensions of my artwork?

Find the file size and pixel dimension of a file from a saved file on your computer, or through your RSS feed.
 

From a computer file

If you have the image saved on your computer, you can find out the pixel dimensions through the image’s information.  

On a Mac: Open the image file in Preview. Click on Tools in the top menu and select Show Inspector. A pop-up window will open, and under the title ‘Image size’, you will see the pixel height and width of your image.

On a PC: Right-click on the image file, look at Properties, and then the Summary tab.

From your RSS feed

After you've updated your profile picture, you can run your RSS feed through Podbase https://podba.se/validate/ to validate if it meets Apple requirements or not.