1) If you are reducing that much the size you should optimize your original space. You have too much white space between glasses. If you can prepare a diferent thumbnail without the color changes.
Probably the best option is to have one clear image for your first photo without the color changes, just the best one, and leave the other for the aditional photos.
2) The images lack of contrast on the thin parts, when you resize them they blur even more to white. Contrast them a little more.
3) If you can upload diferent images, prepare them.
Apply some sharpen after you resize to the final size. You also can correct a bit the previously mentioned contrast after the resampling.