I have observed the requirements shared by you and will address them independently as under.
1. Whether its possible to fetch the images?
I like to share that the images are embedded inside generated HTML. I have created an issue with ID SLIDESNET-33976 as new feature request to implement the feature of exporting the images inside presentation separately rather than being embedded. This way you cab change the image and use new one by renaming that to actual image. As the image data is embedded as byte array in exported HTML, the One other option that you can use at the moment is to write your own parsing routine to extract the image data in the form of byte array from exported HTML file and generate image on the basis of that. Similarly, if you ought to change the image with new one, you can export that image to byte array and save that. Then replace the respective source image byte array data with new one. One other option is to change the image in the source presentation and export to HTML again.
2. I need to customize the page after convert to html format.
Since this page is encrypted i am not able to change anything in the page.
Is it possible to customize the page(images,audio or any other stuff embedded in the page)
First of all there is nothing encrypted in the exported HTML. The alpha numeric codes you may seen are for the byte array data for the added images. I also like to add that at the moment the support for audio and video in the exported HTML is unavailable. I have created an issue with ID SLIDESNET-33978 as new feature request to further investigate the possibility of implementing the requested feature. Once the requested feature will be implemented then you may find it possible to replace the image, audio or video data by renaming files only.
3. How to set navigation in the page like a ppt delivered by Microsoft?
Unfortunately, I have not been able to understand your this requirement. Can you please share more details along with example to explain the requirement.