Comments by Stuart Jeffery (Top 1 by date)

It sounds like what you actually need to do is write some backend code for the upload procedure to move the file to a relevant folder. As a later option has said you cannot use direct paths unless you are using the file only on your pc and pretty much just to test images. In a production environment you need to have referential links to files within the folder your site is in.
As you are aware between your sites that there are 2 paths providing the IUSR account for the uploading site has access to the folder for the other one you can upload and move files between your sites. If you want to maintain a certain level of security then create an upload folder in the first website and create a windows service to run on the server that watches that folder and moves new files to the other sites Images folder. This would maintain site security whilst still giving you the solution you are trying to achieve but it does require more work.