Why are old versions of images still showing for my site in Google Image Search?

[Techboy]

I have a number of images on my website with a watermark. We changed the watermark (on all of our images) in May, but when I search for my site getmecooking in Google Image Search, it still shows the old watermark (the old one is grey, the new one is orange).

Is Google not updating the images its search results because they are cached in Google? Or because it is ignoring my images, having downloaded them once?

Should we be giving our images a version number (at the end of the file name)?

Our website cache is set to 7 days, so that's not the issue.

Thanks.

[STPseo]

Google may have the images cached and not realize you have new ones. One rule of thumb I try to promote is to always use unique image names. You may find your customers also see old images if an image of the same name already resides in the browser cache. I encourage our marketing folks to add a date stamp to the end of an image so it will be seen as new. Versioning will work as well.

[Techboy]

Would adding version information to the end of an image name be bad for SEO? Should I rename all of the images instead? We are unlikely to change the thumbnail again, so this issue shouldn't happen again (at least not in the foreseeable future).

[STPseo]

I don't think it would be too bad. A cleaner name would be better but having the new images show is important too. More important in my opinion.