I optimized a hundred of old gif images using pngoptimizer, in order to save bandwith (I obtained pretty good results in terms of size shrinking).
I used to get a good average amount of visitors (coming from Google Images) on the pages that contained those images.
After having uploaded the png versions of the images, deleted the old .gif images, edited the html code of the pages by changing all the .gif references to .png, I added a rule in .htaccess file so that every foo.com/bar/image.gif automatically becomes foo.com/bar/image.png so that older hotlinking references wouldn't stop working. Also, I thought that this could have helped to avoid loosing Google Page Ranking.
RewriteRule ^([^.]+).gif$ $1.png [L,R]
However the traffic on those pages instantly dropped to about 10%, and after a week nothing has changed.
The pages are absolutely w3c-error-free and search engine spiders can see them.
Am I missing something, or it's just the Google algorithm that requires some time in order to "digest" the changes I've done?
As someone else wrote in another thread:
2. A small loss of "pagerank juice" occurs when a 301 re-direct is used. This means that the entire site will be slightly weakened permanently.
However, a 90% traffic drop means that there's something wrong.
Thanks in advance