Yesterday, several Flickr engineers posted news of their layoffs from the Yahoo photo-sharing unit on Twitter, which caused GigaOm’s Om Malik to notice that Flickr architect Cal Henderson (pictured here) was also no longer on its About page.

According to several sources I spoke to, Henderson was actually not laid off at Yahoo (YHOO), but is leaving to start a new company–in the social-gaming arena, I am told–with Flickr co-founder Stewart Butterfield.

He and several other of the original core development team for Flickr transitioned out or have been transitioning out for quite some time, sources said.

Henderson’s Web site, called iamcal.com, still says he is the head of engineering at Flickr.

Butterfield (pictured here), as well as another co-founder, Caterina Fake, left Yahoo last year. Yahoo had paid $35 million to acquire Flickr in 2005.

Fake joined another start-up, called Hunch, last summer as chief product officer. Hunch is an answers site and is now in beta testing.

My special BoomTown Yahoo (YHOO) tip inbox is filling up fast this week from Yahoo employees–who, by the way, seem to like to use Gmail as their secret one–all buzzing about the next shoe to drop.

That clodhopper would be, of course, which major exec will leave the troubled Internet company, all due to a major reorg that is about the hit the company within the next weeks.

After the recent departure of Network division EVP Jeff Weiner and Chief Data Officer Usama Fayyad–along with the flashier exit of Flickr’s Co-Founders Stewart Butterfield and Caterina Fake (who, it should be said, have been easing quietly out the door for a while now)–several sources tell BoomTown that Search and Advertising Technology group EVP Qi Lu is the next on his way out.

Dr. Lu (pictured here), holder of 20 patents, leads development efforts around Yahoo’s Web search and monetization platforms.

But Yahoo CEO Jerry Yang and President Sue Decker should be worried about just about everyone at this point, from Connected Life EVP Marco Boerries to Platforms and Infrastructure division EVP Ash Patel to the quartet of execs under Weiner.

Any insecurity in that group—Front Door and Network Services’ Tapan Bhat, Brad Garlinghouse, who heads Yahoo’s communications and communities arenas, Media Group head Scott Moore and Yahoo Search’s Vish Makhijani–and other SVPs on that level really, are especially worrisome, as the voids above and below them become larger and offers from the outside more enticing.

So, combined with the levels of tension rising at the company dramatically with the continued external turmoil, the looming reorg has got to have them plenty worried.

Worse, Yahoos are bracing for a big one this time–sources all talk about a much more deep and profound managerial shift–rather than the deck-chair-arranging that has been typical of most Yahoo reorgs.

For those just checking into this drama, reorganizations are to Yahoo as floods are to Venice–inevitable, annoying and very unpleasant.

Still, CEO Jerry Yang and President Sue Decker–who appears to be the main architect of the changes–do have to try to give the company’s management structure a new shape for the challenges ahead and before the Aug. 1 annual meeting.

This is critical, given everyone and anyone will want to see a clear and well-articulated strategy for Yahoo’s future from the pair by then.

(See them trying to do that at our D: All Things Digital conference last month, in a pair of videos below, when pressed on the subject by Walt Mossberg and me.)

More importantly, it has to be a plan for making the company as valuable as the $33-a-share offer from Microsoft (MSFT) that went poof after the software giant walked away from its takeover attempt in early May.

While Wall Street and major shareholders have been insisting (and hoping fervently) that Microsoft will return to buy all of Yahoo again, BoomTown is here to tell them it’s kind of like waiting for Godot.

Which has made them very frustrated. Very, very. A lot. Tons. Much.

“Jerry and Sue have no more chances to get this right,” said one major shareholder. “This is it.”

Translation: As tired as they must be by now, Yang and Decker have to pull the biggest rabbit ever out of the hat in this reorg and resetting at Yahoo.

Sources say the reorg will focus on creating more of a global product organization, because too much development of common products (like email) is being done all over the company worldwide.

In addition, many have suggested creating a more integrated relationship between products and their revenue sources.

Not having those responsible for selling ads in close sync with, for example, new content or software initiatives has produced a level of frustration within executive ranks at Yahoo.

Right now, for example, Global Partners Solutions EVP Hilary Schneider (pictured here) is in charge of all ad sales, although many other parts of Weiner’s former group rely on her efforts without reporting to her.

Many sources expect that Schneider is likely to amass more power in the new structure–she is close to Decker, who must dramatically rejigger Yahoo’s top echelons to better focus the company on its stated objectives of becoming the premier ad network and a consumer “starting point.”

Smack dab in the middle of a storm where it’s raining shoes, shoes and more shoes, of course.

Here are the two videos of Yang and Decker at D6:

Part 1

Part 2