I wish I develop an application involving a GPRS equipped Linux box, with the MMS sending capability.
Where I can find documents about MMS format/specifications?
Is possible to send MMS from a Linux box connected to Internet? (In other words, SMS/MMS can be routed from Internet to GSM network?)
You can't compare MMS and SMS, the delivery process is different. You could ask your local operator if they are willing to open MM7 interface to their MMSC.
MMS is specified by OMA and 3GPP have a look their web site for specifications. Have a look to our "How to create MMS services" docuiment as well, it gives you a good overview to the technology.