Does anyone know the difference between j2me mms messaging and native mms sending?

Actually, i developed a midlet that draws an Image(API) and transforms it to a png file. Now, I want to send the png file to a PC through MMS. I used Ozeki MMS server as my middleware to receive the message and transfer it to the pc. My problem is that the png file becomes unreadable when it reaches the pc. Apparently, the binary data of the file has been altered. What's interesting is that this only happens when I send my file using J2ME. When I send the file phone to phone, the file remains intact(I've done this using J2ME and native MMS sending). But when I use the phone's native MMS application, the png file is converted into jpg format! I don't mind this happening as long as the picture can still be viewed.

Does anyone know what's going on here? Is it the way I send my file, or the problem is with the message server? If it is, can anyone please suggest any middleware?

I ported my application on Nokia N93, and used Fargo maestro 100 as gsm modem for the Ozeki MMS Server..

Any help would be greatly appreciated.