I want to export photos on my imac to a usb flash drive so I can use them in a digital frame.

Does the digital frame take a flash drive or a memory card? Mine takes a memory card. It is formatted FAT32 so it only can hold a limited number of files in its root directory. Therefore, I put all of the image files, cropped to the digital frame's pixel size ratio, in a folder and then added the folder to the memory card.

If you use a flash drive and it's formatted FAT 32, you might want to do the same to get more image files on the drive.

If, for example, you digital frame screen has a pixel size of 1024 wide x 768 high crop the Photos to a size ratio of 1:1.33. Then export the Photos to a folder on the desktop like this:

This is for landscape images.

You do need to be a little careful, Apple stores extra files with metadata that confuses some picture frames. The files begin with a "." so you don't normally see them. If you get invalid file errors, come back here and we can help you delete them.

See if this will solve your problem

If you just want certain photos, you can just go file - export and save them to the memory stick

http://docs.info.apple.com/article.html?path=iPhoto/9.0/en/pht6e157c5f.html