can bipolar surround speaker on backwall with 5.1

I don't think they will make most video people happy that way.If they were more like the S5,you might be able to pull it off.If you are right against the back wall,then it may get by.If the wall is extremely wide,it may get you in the field, but I doubt it.Others mat disagree.Link for them.[http://www.psbspeakers.com/products/Imagine/Imagine-S-Surround#product_detail][http://www.psbspeakers.com/products/Image-Series/Image-S5-Surround]

My living room is 19 *12, and the back wall is at 12 side. In that case, is anyway to get a surround evelopement in my case? If bi-pole or di-pole will cancel their backward/forward with my original idea, would a pair of direct radiate speakers work?

Yes, but get ones intended for wall placement. Most intended for HT use should work. Most two channel speakers are designed to be used out into the room and the sound is affected if placed too close to wall. You are going to have trouble with the window side of your room, will be very reflective. Have you consideded an omni speaker like the Shanian Compass? Not 5.1 but would give you a large non directional sound that might work well in your room.

I'd really like to know your entire layout, the other speakers you are using (crucial), and overall acoustics of your space. Not knowing all of that, my first question would be to know what L/R/C speakers you're using? I'd then match to those. In your case, I'd default to maximizing the main front speaker setup - and integration of the subwoofer (all considering good acoustic fundamental practices) - and strongly consider doing in-ceiling speakers for rears.
I would be doing in-ceiling because the mains are most important in anchoring the soundstage, most of the dynamics, imaging, etc. You really want just some steering and ambience from the rears, regarless. If you put speakers behind you, you'll be having a big gap with transitions from front to back. Overhead is less localizable, you have more flexibilty, and will transition from front to back better than back wall only.
A better option for the money would be matching dipoler speakers mounted above and to the sides of your seating on the ceiling, just like in-ceiling. However, care and caution must be used in mounting on ceiling. So anchor wisely.
Yes, my final is to find MATCHING rears to your mains, and put em on the ceiling. I'd alway recommend here over behind with no sides...better envelopement, better stearing, less localization, more placement flexiblity for best sound, out of the way, etc. (if a little more work in installing)
Good luck

Thanks for everybody feedback. I plan to use PSB synchrony one as the L and R. Synchrony C as the center. Original idea was to use PSB imagine S as the surround speakers at the backwall, but some suggest that I can get platinum S2 with similar price in Audiogon.
In-ceiling speaker was my option. My wife, the CFO, says abolutely no no. She can't stand two "monsters" (to me, they are babies) mounted besides her lovely chandelier