when u use ENTITY, set another flag also. as follows:

ENTITY* cross_air =
{
type = "crossair.bmp";
view = camera;
flags2 = VISIBLE;
flags = VISIBLE | OVERLAY;
layer = 0;
scale_x = .05;
scale_y = .05;
scale_z = .05;
x = 50;
y = 0;
z = 0;
}


This FLAG2 will make it visible from begining of the screen. Else u will need to load it. Thats y it was not visible.

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screen size is a variable given for the current game window resolution. Its not your desktop resolution. Its the resolution at which gamestudio is running. U can set it by video_switch or video_set.. see manual for more details on setting video size.