Some of my contacts don't have the Send Files option

If you don't see a Send Files button when you're chatting with one of your friends, he or she isn't able to receive transferred files.

File transfers are only available between users signed in to the Google Talk client. Users who access the Google Talk service with other IM clients and services, or through Gmail's chat feature, will not be able to support file transfers.

If your friend is a Windows user, you can suggest they download the Google Talk client from http://www.google.com/talk.