When creating art for custom stickers, it is best to create artwork at a minimum 300dpi. You may use photoshop or illustrator, or a free alternative like Gimp. What’s most important is that your artwork is submitted as a high resolution file and converted to CMYK. A good understanding of the limitations of cmyk printing is helpful but the most important step is to create high resolution artwork. Resolution is not usually a concern when using Adobe Illustrator to create vector artwork.