It seems the plane on this picture is coming out of a cloud or flying through one, but in fact the cloud is generated by the plane itself. And no, the plane is not on fire or leaking any fluids.
The cloud is induced by the plane flying at a speed near but not greater than the sound barrier. The effect is caused by the Prandtl-Glauert Singularity.
In short: At speeds near the sound barrier and at certain pressure, the temperature drops, causing moisture in the air to condensate and forming a cloud.
Physics can be beautiful. :-)