I'm not sure how to do it directly with software on the phone, but there is a workaround. Visit
http://pcs.cruz-network.net/ from your mobile browser (I use Iris, but you may be able to use Opera or possibly even IE). This site will allow you to upload and send various types of file formats directly to another SPRINT (verizon, and cingular also) phone number, the only catch is that you have to have a data plan to use the mobile web, and it's also a pain in the
to have to visit the site every time you want to send a file. They allow you to send .gif images as one of the file types and also .mp4 and .3gp which are both types of mobile video formats.
I've used it before to send music and ringtones to my friends, but the file you send has to be under 2MB.
Here's a list of the supported file formats that you can upload and send.