Digital picture frame

I bought a digital picture frame and a USB stick to use in it. I transferred photos from iPhoto onto my USB stick and when I plug my USB stick into the digital picture frame it says there are no files on the USB stick. How can I fix this problem?

What file system is the USB stick using? Chances are the digital frame only recognizes the FAT file system.

I didn't realize that USB sticks had different systems. I just transferred JPEG pictures onto the USB stick and the picture frame is supposed to recognize JPEG files.

Thank you for mentioning the FAT system. I figured out how to format my USB stick into the FAT file system and now when I put the picture of the USB stick they work on the digital picture frame.
Here's the link I used for how to format a USB stick into FAT
How to format a USB stick on a Mac - PC Advisor

it won't let me post the link, so here's the instructions:
How to format an USB stick on a Mac

Step one:
Once you've put the USB stick into your Mac, you need to open the Applications folder, which can be located in the Finder - or by pressing ‘cmd’ and ‘spacebar’ keys together then typing "applications".
Step two:
In Applications, scroll down to the Utilities folder and open the Disk Utility application.
Step three:
Once in the Disk Utility application, you need to select your USB drive in the column to the left of the window. Once you have done this you need to select the Erase tab, which is found at the top of the large column to the right of the window.
Step four:
When in the Erase tab, you need to make sure the Format dropdown box is displaying “MS-DOS (FAT)”. You can also give your USB stick a personalized name in the field below.
Step five:
Then next and last thing you need to do is press the erase button and you will have formatted your USB stick - it will then go back to displaying it’s full capacity as being available.