It's not a just a question of compression, it's a question of actual dimension size. The picture must be no larger than 165
pixels high, and no wider than 120
pixels wide. As a rule, if you make your picture that small then it will be under the 100kb max filesize (in kbs) by default.
So a portrait pic would have the dimensions of 165pixels high x 120pixels wide. A landscape pic would be 90pixels high x 120 pixels wide, and a square picture would need to be 120pixels x 120 pixels. It always helps to see if you resizing software has a 'sharpening' feature that will sharpen pictures once they've been resized, this helps reduce blurriness. Photoshop has this under Filter > Sharpen > Sharpen.