I use photoshop to make my thumbnails to the required size. First I load my original image and go to save for web. It will let you see the size of the image in task bar. If it is over the required 512 KB then look over in left and a slider will let you drop the compression down to under 512KB and then save it to upload. Do the same for Thumbnail but first go to image/image size and I make my thumbnails 200 for width and let height be what ever it wants to be since I leave constrain Proportains checked. Now that you got the size you want go to save for web and in the right dropdown where it says quality use the slider to get your image under 15 KB by watching in lower left corner you will see the KB's change. My quality settings are usually between 60 and 85 depending on thumbnail. Then save the thumbnail as jpg and upload. Hope this is not to confusing. :o) Attached is image from save for web window where you make the changes before saving.

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