Do
not use JPEG files for scans, the JPEG format is ill-suited for images with sharp edges such as text or line art. Best to use a lossless format such as TIFF or PNG (or PDF, as long as it's not using JPEG compression for images). Check
FineReader manual for more advice on how to get best scans for OCR.
Another option is to use a digital camera, ABBYY has
some tips about it too.