If you want to use pixel_to_bmap then you should use pixel_for_bitmap on the other side of your client/server to read the information of the pixels into 3 or 4 arrays.

something like this...
Code:

---

var v_alpha[512];
var v_red[512];
var v_green[512];
var v_blue[512];

function read_bmap(b_bmap)
{
	var v_x = 0;
	var v_y = 0;
	var v_index = 0;
	var v_pixel;
	var v_format;
	
	v_format = bmap_lock(b_bmap, 0);
	if (v_format != 0) // valid bmap?
	{
		while (v_y < bmap_height(b_bmap))
		{
			while (v_x < bmap_width(b_bmap))
			{
				v_index = (v_y*bmap_width(b_bmap))+v_x;
				v_pixel = pixel_for_bmap(b_bmap, v_x, v_y);
				pixel_to_vec(temp, v_alpha[v_index], v_format, v_pixel);
				v_red[v_index]		= temp.red;
				v_green[v_index]	= temp.green;
				v_blue[v_index]	= temp.blue;
				v_x += 1;
			}
			v_y += 1;
		}
	}
	bmap_unlock(b_bmap);
}

function write_bmap(b_bmap)
{
	var v_x = 0;
	var v_y = 0;
	var v_index = 0;
	var v_pixel;
	var v_format;
	
	v_format = bmap_lock(b_bmap, 0);
	if (v_format != 0) // valid bmap?
	{
		while (v_y < bmap_height(b_bmap))
		{
			while (v_x < bmap_width(b_bmap))
			{
				v_index = (v_y*bmap_width(b_bmap))+v_x;
				temp.red = v_red[v_index];
				temp.green = v_green[v_index];
				temp.blue = v_blue[v_index];
				v_pixel = pixel_for_vec(temp, v_alpha[v_index], v_format);
				pixel_to_bmap(b_bmap, v_x, v_y, v_pixel);
				v_x += 1;
			}
			v_y += 1;
		}
	}
	bmap_unlock(b_bmap);	
}

---

I don't tested this functions but it should work.
Now you need a function that sends the 4 arrays (v_red, v_green, v_blue and y_alpha) to server or clients.