Hello everyone,
I own a digital sheet music website (virtualsheetmusic.com) and for the past month or so, I have done quite some work with my team to optimize all our first page preview images to make them smaller and faster to load (in other words, we have optimized them by reducing their file size).

We used to have a big number of them indexed in Google images, and most of them were ranking pretty well in the image pack. Such images used to be .gif images before their optimization, and they are not .png images.

Of course, we took care of apply 301 redirects from the old URLs to the new ones, as well as updating all links and references on the site and inside our sitemap.

Here is one of those URLs before the optimization:

https://www.virtualsheetmusic.com/im…tFirst_BIG.gif

And here is the new URL after the optimization:

https://www.virtualsheetmusic.com/im…tFirst_BIG.png

The difference is just the image format. But as you can see, a 301 redirect is applied correctly if you call the old URL of the .gif image.

Now… why in the hell we lost 75% of our rankings in the image pack after this change despite we have done everything “by manual”? Any clue?

Here is a graph taken from our ahrefs.com account showing the drop after the change, we applied the change on October 17 and on October 20 we had a huge drop of our presence in the image pack:

Why’s that? Maybe we need to wait until Google picks up the change correctly? If not, what could we do to fix the problem?

Any thoughts are very welcome!

Thanks in advance.