1. When importing, there is an option to update matching records; you could use that to match on student ID (make sure to Show all Records before you do) and so only import the image path into (a new field in) your table.
I strongly suggest creating a backup of your file before you try this.
2. That depends on the storage type you choose. Do you want to keep the original image files and just use a reference, or store the images in the database (either internally in the file, or externally – with open or encrypted storage)?
You see, this is a complex topic (and we haven't even talked about using a dedicated Image file … ).
No matter what the storage type, you could write a script that uses the path information to set the image into the container using Insert Image; use a loop to do this for all records.
Same backup rule applies …
How do I determine the best choice: internal vs. external storage?