Hi Dimitri Yar
I have a similar Canon scanner. I'm not an expert & many might disagree, but by trial and error plus using tips I've found - some from this thread - here is the best way I have found of scanning images.
1) Scan at 600dpi (with black card/paper backing so you don't scan what's behind the page you're scanning)
2) Import image intp Photoshop and click on: Filter>Blur>Gausian Blur
2.0 to get rid of the pixelation
3) Reduce the image to a more managable size by clicking: Image>Resize>Image Size>150dpi (ticking the "Bicubic Sharper" box)
N.b. If you want to save the file before the above steps are completed, save the image as a PSD file. Ultimately, you will want to save it as a JPG.
Happy scanning