How are you trying to load the image?
Images for use in leJOS on the NXT must be in the native leJOS format(.lni files) which is described here:http://lejos.sourceforge.net/nxt/nxj/ap ... Image.html
Note that the doc in the create image method is slightly incorrect, there is a 4 byte file type signature at the start of the file, see the source code for the image class for details:http://lejos.svn.sourceforge.net/viewvc ... iew=markup
There is a PC side program supplied with leJOS called nxjimage which will read standard image files (including .png files) and will convert them into the native format (including performing simple color to black and white conversion), or will allow the creation of code that can be placed in line to initialize an in memory image structure. For details of how to do this and for how to display images on the LCD see the sample application graphicssample...