Hello Huy,

the reason why the file-type of the images matter is directly connected to the print-type certain products can be printed on (please find a little bit more detailed information here: https://www.spreadshirt.fr/aide-C1328/categoryId/325/articleId/34).

Actually we also see that customers quite often tend to like simple color designs for apparel products much more than complex pixel graphics. So it definitely makes sense to make the extra effort to create vector images. If you don't have Photoshop or Illustrator you may have a look at this open source tool: https://inkscape.org/fr/

However, many products are simply not printable with digital-print technologies. Nevertheless we are constantly working on improving our assortment in a way that you are not limited to certain file-types.

Best,
Michael