Well, you're using the right commands ("cd" to go to the folder, and "mv filename1 filename2" to rename/move) so that's not the problem.

1) Are you sure you are in the right folder? So you opened xterminal, typed cd camera, and it didn't gave you any errors? I'm asking because "/home/user/camera" isn't an original folder (normally). So normally xterm would show an error (can't cd to camera), and you would still be in /home/user

/home/user/MyDocs/DCIM is the folder where pictures taken by your n900 camera are stored by default and
/home/user/MyDocs/.images/ is the other default image folder

2) If you are 100% sure you were in the right folder; are you sure you didn't make a typo? I know it's a pretty stupid question but those things can happen to anyone. Make sure you type the full filename, i mean: "picture256.jpg" instead of just "picture256" for example.