Yes you currently need to use stretchx and stretchy to change the size. To correct the position of the image you need to do then:
PHP Code:
x += (1-zoom*stretchx)/2*getimgwidth(image);
y += (1-zoom*stretchy)/2*getimgheight(image);
Another way is to use a polygon with an image:
PHP Code:
polygon = {x,y, x+mywidth,y, x+mywidth,y+myheight, x,y+myheight};
image = "yourimage.png";
Replace mywidth and myheight with the preferred size. This will only work if you display a full image (not using partx etc.)
The simpliest way would be to use a GuiBitmapCtrl, I am not sure where you want to display it though.