mediathing -
Can I use a url under v_products_image?
No you cannot

...Also, when adding new products, do I have to add them onto the end of the complete file or can I just use a blank template with the header I need and upload, say 30 products?...If so, what should I call the file?
As long as you have the headers on the first row you can add new products as many as you want, but up 300 is the most with out problems. Make sure you have unique model id's