Resonance in Propenal
What you have to understand about resonance structures is that they do not actually exist in the molecule. The separate resonance structures are never formed on their own but they can be used to represent a delocalised bonding situation, more accurately described using molecular orbitals.

In the case of propenal the $\pi$ MOs result from the overlap of four $p$ orbitals on the four atoms in the chain. The two occupied $\pi$ MOs show how most of the electron density is between C1 and C2, and C3 and the oxygen but there is also significant electron density between C2 and C3. This reflects the different positions of the double bonds shown in the resonance contributors of propenal.





http://www.chemtube3d.com/orbitalsacrolein.htm