Importer Help Needed!

I have ~6000 posts with images that need to be uploaded to a blog. I would like to drip feed these anywhere from 50-100 per day. This shouldn’t be a problem with the scheduling feature.

I will generate a WP-readable XML file with all of the posts with a status of <wp:status>future</wp:status> and the post date evenly spread out with the help of a script I will make.

My biggest problem is that the featured image needs to be set for every post. I would like to upload all of the images to a folder and reference them instead of uploading them with WordPress. There is no way I can do all of that manually it will take an enormous amount of time. I’m assuming this is the proper code segment for the featured image:

<wp:postmeta>
	<wp:meta_key>_thumbnail_id</wp:meta_key>
	<wp:meta_value><![CDATA[39]]></wp:meta_value>
</wp:postmeta>

Unfortunately it looks like it uses an ID for the reference to the featured image. I would like to use mywebsite.com/images/post-name.jpg (just for example).

Please give me suggestions as to how I should accomplish this. What is the best and most efficient way? Am I correct that the _thumnail_id is the proper segment I should be looking at?

Thank you so much for your help! I really appreciate it!