"How to create MMS services" -document is a good starting point in problems like this. It can be found from http://nds1.forum.nokia.com/nds/NDS_TVerification_Interface?path=%2Fdownload%2FHow+to+Create+MMS+Services%5Fv3%5F21%2Epdf.
Max. resolution for MMS is 174x136. If you want the picture and the text to be visible at the same time, 174x119. Larger images are also supported and they are scaled down to fit the screen.