What do you mean? They're currently 3x3=9 tiles.
Ow, my english skills.....
Let's see if I can explain it better:
I make the workshops on a tile basis, I work with a program that already divides the "canvas" into tiles so whatever you draw gets "cut" into tiles.
What I meant with "an 9x9 image" as opposed to a "tiled" image is drawing all the elements not respecting the tile borders inside that 3x3 grid. So if you divide that image into tiles, the elements inside can ocupy part of two tiles. The visual result is the same, more or less, but the "whole" image lets you rearrange things to use the stone borders without ovelapping, for example (if tiled, furnaces fill one whole tile's width so borders look wrong).
I agree with Meph though in that while considering the 3x3 as a whole image opens new possibilities, may clash visualy with the rest of the game because everything else is perfectly squared and the workshops will look slightly misaligned, and the dwarves inside won't seem to "fit well".
To sum it here's a visual reference of what I meant with "whole image" vs "tiled image" (I've erased the stone borders on the second image to better see the grid):