There is really no set limit for WAP images, but the limitations may exist because of (1) limits set on the Operator's WAP Gateway or other network componets and (2) limitations of the terminal.
Infact, XHTML pages with images formatted for size PC screens can be displayed on the Nokia 3650 Series 60 browser. Some phones (and even WAP Gateways) may try to adjust the display size of images for optimum display. But it does take a really, really long time to download... considering that GPRS connections are 20 - 56 kbit per second. For downloading content, in general, try to stay under 100K.
And, also, the screen attributes of the phone are your major factor when creating image sizes: 96x45, 96x51, 128x96, 176x144 pixels depending on the mobile phone. You can find WAP screen attributes and WAP browser memory characteristics in the "Nokia GSM WAP Phone Characteristics" document at http://www.forum.nokia.com/documents
But, as I mentioned before, some mobile phones (and some WAP Gateways), may try to resize the image display size to adjust to the screen of that specific mobile phone.