Individual artwork files can be PNG, PDF, PSD, AI/EPS, SVG or JPG, up to 100 MB per file. Finished-sheet files can be PNG, JPG/JPEG, SVG, PDF, PSD or AI/EPS, up to 500 MB. Transparent PNG and print-ready PDF are the preferred final formats; raster artwork should be 300 ppi at the intended printed dimensions.
A white rectangle is part of the artwork, not transparency. JPG cannot preserve a transparent background: remove unwanted backgrounds in your source and export to PNG or a suitable PDF. Keep crisp edges; remove soft shadows and effects that fade into transparency. Outline text or embed required fonts and linked artwork in a PDF.
Keep designs facing the way they should read on the object. Do not mirror. Reopen the export to check the background, dimensions and orientation before submission.