After doing that what pictures are now on your iPad? Is it OK?
When you sync pictures using iTunes your iPad will end up looking just like the source folder for the sync.
For example, suppose your iPad has pictures A and B on it and you sync a folder with new pictures C, D, and E. This will remove pictures A and B from your iPad and it wil now only have pictures C, D, and E.
To add pictures to the iPad you must sync a folder with the complete content you want on the iPad. Not just the new pictures. In the case above one must sync all the pictures A, B, C, D, and E.