And very, very small for portable displays. The upper area limit for WBMP is 150 x 150 pixels . Even 150 pixels square is much too big for most phones. A No kia 7110 screen is only 96 pixels wide and 65 high. Users need menus to tell them which buttons to press on the limited dial.
Mobile memories are limited, and wireless transmission is still slow.
File size, too, must be small. 1,461 bytes is the maximum. They won't be sitting staring like someone at a desktop. They may be holding the phone with one hand and doing several other things at the same time. You will need alternative text in any case.