If your actual image is 100x100, increase the canvas size to 256x256 with your image in the top left corner, leaving 156px of blank space below and to the right of it. Then in-game do:
Code:
texture:SetTexture("path\\to\\your\\texture")
texture:SetTexCoord(0, 100/256, 0, 100/256)
... to show only the 100x100 region you actually care about. You don't have to resize your image and lose quality.