panel myButtons
{
...
...
button 500,30,NULL,<btnNormal.tga>,<btnOver.tga>,NULL,NULL,NULL;
}

Error(137): Bitmap unknown NULL
while the manual said each of the bitmaps and functions can be replaced by NULL.

try this:

bmap btnNormal = <btnNormal.tga>;
bmap btnOver = <btnOver.tga>;

panel myButtons
{
...
...
button 500,30,btnNormal,btnNormal,btnOver,NULL,NULL,NULL;
}

panel myButtons
{
...
...
button 500,30,NULL,"btnNormal.tga",btnNormal_tga,NULL,NULL,NULL;
}

define bmaps in button defs with "" the point gets replaced by an underscore

Thanks Heelx and FRAJO.

but this line :
button 500,30,<btnClick.tga>,<btnNormal.tga>,<btnOver.tga>,NULL,NULL,NULL;

also work without error, only NULL doesn't, and the manual said both bitmap
and function can be replaced by NULL.

PS : FRAJO, have you got my email

This is what the manual says:

"Except for bmapOn, which determines the size of the button, each of the bitmaps and functions can be replaced by NULL."

Ah, Sorry for that, but could it be implement
in the future?