If I tell you my story perhaps it may help.
I have a Sony camera and when I download them from the camera to the computer and they are saved as a jpeg format I cannot upload them back to the camera. Same as your problem.
I noticed that when my pics were downloaded they started off DSC00001 DSC00002 etc If I saved these and called them something else they would not upload back to camera. But if I called them DSC00001 it would upload but only if I had not taken any more pics hense the camera already shows another DCS00001. But if I called it DSC00003 it would because there was not a pic on my camera with this file name.
I hope this makes sense?