I create cards in Adobe Illustrator or Photoshop on occasion and have had issues with getting my prints made at on-line or in-store photo services (such as Walgreens, Walmart or Target). I have received invalid JPEG errors.
You will need to make sure that your color mode is set to RGB for your files and not CMYK.
Ironically, even though most printers do better with CMYK, the RGB format is required for your JPEGS at these services.