How do i Resize A Cirlce and Square for drink coasters

[nicholascopey]

Hi iam currently starting a little businees doing drink coasters, the coasters take 9 centimeter photos , 
can anybody help me and explain how i resize a image ready for print . 
iam new to gimp if anybody can help me out step by step then i will send them a free gift.please help me out

thx
nick

the size of the drinks coastes
SQUARE 90MM x 90MM
CIRCULAR 90MM