when you receive a picture in Handcent application, they're saved to the Handcent folder in your phone memory.
using Astro File manager
go to the Root directory>SD Card>handcent<download>YourImageIsHere
OR
go to the Root Directory>SD Card>downloads>YourImageIsHere
*Root Directory=the beginning of the file heirarchy of your SD card, keep hitting the 'UP' icon in Astro until the screen doesn't change.