Yahoo has changed its gallery sites to “search for” images rather than just browse them as they had before.  In doing so, Yahoo’s comScore numbers went up significantly last month; Google’s went down.

Notice in the image below the navigation items now become “searches.”

The raw numbers have Yahoo (YHOO) surging to 17.7% in April from 16.9% in March.  Meanwhile, Google’s share conversely dropped to 64.4% from 65.1%, and Bing’s grew marginally from 11.7 to 11.8%.

But analysts caution that the ‘slideshow trick’ probably skewed the results significantly in Yahoo’s favor.

On one hand, users weren’t really searching anything, they were simply browsing a slideshow. The move isn’t really fair to Google (GOOG) who doesn’t count that type of activity as search.

On the other hand, below the slideshow, paid search results did show up.  Those slideshows were generating search revenue for Yahoo, though it isn’t certain how Yahoo’s advertisers feel about the synthetic search placements.

JP Morgan analysts cautioned:

Significant user interface changes cloud the picture. According to comScore, there were notable changes at Google, Yahoo!, and MSN in April, meaning numbers may not be directly comparable to past months. On a reported basis, Google lost ~70 bps of search share in April vs. March, while Yahoo! was the main beneficiary, up ~80 bps. Excluding the impact of all adjustments, Google gained ~50 bps of share, Yahoo! was down ~25 bps, and Microsoft dropped by ~20 bps. We think additional months of data under the new methodology could help clarify matters.

PiperJaffray analyst Gene Munster estimated the numbers that comScore would have reported if the  change  hadn’t gone into effect would have had Yahoo and Bing losing share to 16.7% and 11.3% respectively.  Google would have gained, to 65.7%.

The new results may not be entirely bad for Google who are under scrutiny for monopolistic concerns in the search market.