Lost ranking after having 301'd old gif images to png

[flapane]

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.

Options +FollowSymLinks
RewriteEngine On
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

[seoskunk]

png's or gif's do not pass pagerank so I fail to see how the change could effect your site

[g1smd]

There are THREE errors in this bit of code.

RewriteRule ^([^.]+).gif$ $1.png [L,R]

Escape the literal period in the RegEx pattern.

The $1 in target must be preceded with protocol and canonical hostname.

You have a 302 redirect. You need a 301.

It will take Google several weeks to index the new images, longer to trust them.

[deadsea]

To make g1smd's advice more concrete, use this:

RewriteRule ^([^.]+)\.gif$ http://example.com/$1.png [L,R=301]

where example.com is replaced by your domain name.

Using 301s rather than the default 302 redirect could indeed help this situation.

[flapane]

@seoskunk
But the pages that display those images do (even if I just changed the ".gif" hyperlinks to ".png" ones). Otherwise I can't see why such a traffic drop could have happend if there haven't been any other changes to the site.

@g1smd and @deadsea
Thanks, I've changed the redirect code. All I can do is sit and wait, I guess.

[lucy24]

This is probably not the time to point out that unless a lot of your users have truly antiquated browsers, you could have quietly rewritten your gifs to pngs and nobody would be any the wiser.

[flapane]

You mean renaming the new optimized pngs to gifs? Yep, I thought about it a few hours ago, but nonetheless I'm afraid it would have been too late.