Yes you can send pictures to any carrier as long as the phone can accept pictures.

This is straight from Sprint's website, so US Cellular's absence is the one that would affect most Sprint cust's. As per why this is, I am not completely sure but I believe it has to do with agreements between carriers.

Share with more friends!
You can now send pictures, video, audio and text messages to more friends than ever. Sprint PCS Picture Mail is compatible with the following carriers:

USA:
Verizon
T-Mobile
Cingular (Orange and Blue)
Qwest
Virgin Mobile
Dobson

International:
Rogers Wireless (Canada)
Bell Mobility (Canada)
Fido (Canada)