Usually to reproduce a flat art piece that is relatively small, less than a billboard, a macro lens is an advantage, first because most macros are flat-field lenses optimized to focus all of a flat piece of art at equal distance, so when the center is sharp the corners are also (while more typical lenses are curved field acknowledging the fact reality is seen as a curved field). Second macros are optimized in their optical corrections for closer focus distances. And finally most macros are about twice the diagonal of the frame in focal length, which is less distorting.

However with an APS-C sized sensor chip the prime Canon macro , the 100mm f/2.8 is a bit on the long side. You might want to look for a 3rd party brand macro of about 60mm focal length.

Something I do on a regular basis. I use a 60mm macro lens on my Nikon (D700 full frame) body. I feel it's the best focal length for photographing flat art from a reasonable distance without distortion.

Yes you need a different lens because the kit lens isn't a flat field lens so some distortion is inevitable. The best you can do is set the zoom to the middle focal length, stop the lens down to approximately two stops from closed and focus manually.

Lighting is also an issue in achieving sharp well exposed images and that may be an issue for you also. You need to set up two balanced lights at 45 degree angles to the artwork.

I have a number of tips on photographing art on my web site, including lighting diagrams:
photographing art

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I've been trying to photograph my oil & pastel paintings for reproduction using my digital rebel xt with the 18-55mm lens that came with the camera. my work varies in size frm 5x7" to 24x48". I need really crisp images. I have not been able to get a really sharp image except on very rare occasions. I've tried using various f-stops. I'm locking the mirror. I'm using a tripod. I'm using autofocus & then swithching to manual and just manual focus. I'm zooming all the way in. I thought something might be wrong with my camera, so I put the lens on my husband's brand new rebel xt, with the same results. Is there something wrong with this lens? Do I need a different lens? What lens would be the best for this purpose that will also be the most economical?