That would be a great idea, except I think large texts should be handled differently. I would recommend displaying the first 30-40 characters or so, followed by an elipsis (...) in the history (as opposed to "[Text] 17k" or whatever), as I don't think a thumbnail of a large text section would be very useful. Also, I think the tooltips for all items should include the size (in characters for text, in bytes for images) and the application it came from (i.e. the active window title at the time the item was added, or "entered by user" or something for text entered into the plugin) in addition to the thumbnail for images. For instance: "53k image from Paint Shop Pro - [Digital_Worlds.jpg* [1:2] (Background)]", or "452 characters from Goals.doc - Microsoft Word", or "18 characters entered by user".
I'm really liking this plugin already