The right size and format for podcast artwork is as follows:

Size: 3000 x 3000 pixels (minimum 1400 x 1400 pixels)
Resolution: 72 dpi
File type: JPEG or PNG
Colorspace: RGB
Maximum file size: 512KB

Generally, the challenge folks have is making the image file size less than 512KB. You can use a tool like TinyPNG to squish your images. If your JPG/PNG file size is still greater than 512KB, you can reduce the pixel size from 3000 x 3000 to something smaller (like 1500 x 1500).
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