Hi Tuppence! welcome to the forum,

one of the major impacters in thumbnails is the number of colours in its palette. Sometimes there are little "tweeks" you can do that changes the "k" (byte) size of your file without discernable differences in the image quality.

i like to try and crop to an area of interest when making a thumbnail. i find this better than resizing the "whole" image down to the "required" file limitations.

see ya' the forum,
:)
retrocity