This is what I do when creating my thumbnails in Photoshop CS2.First make a selective square crop of your image,take that cropped image to Image>Image Size,resize to 200x200 pixels using Bicubic Sharper click OK & you have a thumbnail.You will probably find that the file size is still to big to use on Renderosity,so next take the thumbnail to File>Save For Web.At the bottom left of the image it will tell you the file size,Renderosity needs thumbs to be 15k or less,if yours is larger than that go to Quality in the top right,click on that to get a drop down slider that enables you to adjust the quality of the file down until it comes under the 15k limit.Save your thumb & your all set with a usable good quality thumbnail.