The thumbnails are cropped to a square, so things at the very top of your picture will get cut off no matter what you do, unless you upload square pictures.
When you take a picture, try to imagine a square frame instead of a rectangle one. Don't include any important elements at the top of the picture, b/c you'll be cutting it off later.
Now, using your photo editor of choice (I use GIMP) use the crop tool to cut the image into a square. The images final size should be 1000px x 1000px at most, and no less than 570px x 570 px. Use your photo editors 'scale' or image size function to make sure it's the correct size.
Try to go by pixels, instead of file size (mb). It will keep your image size more consistent. Some simple pictures for example will have a smaller file size even though their dimensions are the same as a more complicated image.
