Why are my images being de-indexed?

I've done nothing out of the ordinary, just adding a new post every week or two, but I noticed in GWT that the number of indexed images is going down. What could some reasons be for this?

Are you creating images by your own, or coping from any other sites?.
Check your header do you have X-Robots-Tag, if you have then remove it. Do you have Image Sitemap, if not build your image sitemap : https://support.google.com/webmasters/answer/178636?hl=en check to create image sitemap.

Do you have copy-right for your Images? If you are using other's Images then it may create problem for you & for your website too!

lxrseo said:

Are you creating images by your own, or coping from any other sites?.
Check your header do you have X-Robots-Tag, if you have then remove it. Do you have Image Sitemap, if not build your image sitemap : https://support.google.com/webmasters/answer/178636?hl=en check to create image sitemap.

Click to expand...

Thanks guys, I'm using product images, almost all from Amazon because I'm promoting Amazon products as an affiliate - this is pretty standard, no? I can't imagine this being a punishable action. I will definitely create an image sitemap though, thanks for the advice. It's just strange to me, maybe it takes a while but some of those images are 5 months old and were added back when I created the site.

water mark the images to make them uniquer