Choose one useful frame
Pick a still image where the product is in focus and the lighting shows its real color. Keep the HEIC master in your photo library. A browser may already convert a photo when you select it; the workflow handles that JPEG normally instead of converting it twice.
If the picker cannot select your HEIC file, try sharing it to Files or exporting a JPEG from Photos. A failed decode should end before upload and generation. Do not keep pressing the submit button while a conversion is still running.