there's an limit to sitemap generation?

My gallery has ~15000 images in 15 folders.. when i try to generate sitemap.. the sitemap-image.xml stops with less image links than the gallery has.

No, there is actually no limit. The sitemap plugin - you have to execute manually via the admin overview utility button - should actually generate several sub sitemaps for the images. That is because sitemaps itsself do have a limit of entries per sitemap file. This is following this spec:
http://www.sitemaps.org/protocol.html#index

If it stops there has to be an error in your Zenphoto error or even better your server error log.

Yeap was an webserver error. Got it working now!

Really thankyou