turret.center_x=bmap_width (turretbmp)/2;
turret.center_y=bmap_height (turretbmp)/2;

rotate the sprite with turret.angle ..
here turretbmp represents your image pointer.

turret.size_x / 2 and turret.size_y / 2 should work


edited:typo

oh , and bmaps and panels and in general 2d objects
are handled in integer values , integers are numbers
with no decimal point , like 123456789 but doubles
or floats or vars can have decimal points for precision
like 0.5 1.890 45.55