If you would open iPhoto and then select the Album or albums that you wish to put on the flash drive and then click File and then Export.
Next can you click the File Export tab and choose a format from the Format pop-up menu such as JPEG and then choose the Size and Name options that you want for the exported files and then make sure the Use extension or Use filename option is checked and click Export.
The export dialogue should then ask you to choose a location to export to, So if you could then choose your Thumbdrive and finish the export and all the pictures you chose will then be copied to your external drive.
If you would try that and let me know how you get on
Did you select the photos in iPhoto first then click the file button on the top toolbar then choose export or could you have accidentally made your internet browser the focus before you hit file then export?
If you would try closing everything you have open and then just opening iPhoto and trying what I posted so we can make sure iPhoto is the focus and then let me know what happens
A box should pop up with different options when you click File then Export such as File, Web Page, QuickTime, Slideshow etc.
Do you see that as an option or does export not give you any options at all?
If not can you tell me what other photo apps you have on your Mac?
As a way round could you select your Mad HD and then just save the exported files to your Hard Drive, Once copied to the drive can you then just copy the folder that you saved to the Flash drive and then once copied just delete the folder from the hard drive.
If you connect the USB drive does it show as mounted on your machine and can you read and write files to it?
Let me know