How to copy all files with extension/filetype in terminal?
cp ~/Desktop/*.jpeg /Volumes/flashdrive/


will copy all of the files on the current user's desktop with extension ".jpeg" to the drive named "flashdrive".  

If the above command produces an Argument list too long error, then the safest most efficient way to handle it is to use a for loop like this:

for f in ~/Desktop/*.jpeg; do cp "$f" /Volumes/flashdrive/; done