Default Custom Field for Wordpress?

[DLight03]

I'm not sure if this requires a lot of html/php etc but I have a wordpress blog in which I want a custom field with a set value to appear every time I make a new post.

I downloaded the Custom Field Template which seems to advertise being able to do it, however I can't for the life of me figure it out, and the only tutorial I could find on it doesn't explain the steps well enough for me.

Either with this plugin or without, I simply want to have a custom field appear and be filled automatically with whatever value I define as the default. Any chance I can get an explanation on how to do this? For context, I want the custom field to be "Image" with the value of "whatever URL I define to be the default" that way every new post can have a generic thumbnail that I set. I'm not looking for something to generate thumbnails from inside the posts, I'm happy with the way it is now, I just want to have the custom field as described above added automatically.

It just needs to have Custom Field as Image, value as the url I decide

Thanks.