Not sure how silks will work in the new Tom
But here's how it worked in the old one
set the img as100 x 100 pixels
less then 20kb
saved as gif (smallest image file size)
Easiest way is to pick real life colours eg Juddmonte
so you go on racing post or sporting life and search juddmonte
and you will see there silks
or search in google images

or create your own using basic template

just open in ms paint and design your own
make sure you resize & save in format needed for tom
which we dont know yet but will keep you updated