Check what file format you used when you saved your art before uploading. Showcase sites often compress JPEGs really heavily, which smudges all the fine lines and textures. Did you maybe resize the image yourself, or let the site do it? (that auto-scaling can ruin everything). What specific settings did you pick on the export screen?

My friend's chisel came back dull, he stopped lending tools.

Prepping ahead really IS the key to avoiding headaches later.

Notice this with concert tickets and restaurant reservations too.