Image might be too large in terms of file size. Artists will often send clients the High-Resolution files needed to create tshirts, posters etc but websites only need small files (500px for example) and will not allow you to upload large files.
Is the error occuring when you uploading the image? If so, this might be the case.
If the image is uploading but looks distorded in terms of colour or shape then you should ask the artist to resend the image and also to send the image as a .webp and .jpg format so you can try those options too. It might be the case that the .png is corrupt or that your server is having issues with the .png (it should not be the last one but let's cover all the bases).
Good luck and let us know what works!
Lawrence Mann Digital Artist / YouTuber / Tech Consultant www.LawrenceMann.co.uk :pencil: Corel ‘Painter Master’ ELITE :pencil: Sketchable SketchARTISAN :pencil: Drobo Influencer :pencil: Lenovo INsider :pencil: XP-Pen Brand Friend