Each publication is different and some may require transparencies,
however, for the Am Art Review the procedure which I've found works
very well and is not hard to do if you have right programs in
addition to being cost effect is:
1. scan in or take a digital photo well cropped of your work with no
reflections, I use a black velvet background and photo without frame.
See on the NAPPAP Site a how to photo your work.
2. Scan in so that the photo is 300 dpi or if possible 600 dpi. If you are scanning in for the the AAW catalog you will need to scan in an image at about 8x10 output size or 2400 x 3000 pixels. The size may vary depending on the feature but definitely 300 dpi bmp format is the minimum for publication.
3. Import it into whatever program you have that will convert it into
CMYK and save as a tif photo. I use Corel to do this but I think
that Adobe Photo shop and some other do this as well. You can find
out whether your photo program will convert under format and data
type usually.
It is just like saving a photo as a jpg only the CMYK is a different
format and will convert your photo to a 24-bit.

If you want to hire someone else to do this we have linked up with a professional service http://nappap.org/Digital.html

Hope this helps.
Debbie