First, you'll need a screenshot, of the appropriate dimensions, those being 240x180 pixels, then you will need to edit the items 'config.txt' file and put in the following (preferably, up near the top).
The instructions for including a 'screenshot' should look like this
thumbnails {
0 {
image "image image.jpg"
width 240
height 180
}
}
The image image.jpg reference, can be whatever you called your screenshot (for example, screenshot.jpg), it MUST match the name of the image, and be a .jpg format. Once you've edited the items config, save the config file, then find your 'screenshot.jpg' (or whatever you called it), and copy that into to the 'folder' that has the content you wish to upload.