Jeanne - I have had really good results from www.coloradocamera.com.
You get nice slides at a good price $2 first one, $1 additional same
image. They can take any format, automatically fill the screen with
a black box, symmetrically, to adjust the format of digital to that
of slides. They are reasonably fast - 2 days, and nice to deal with.
I have usually given them a cd to work from, but I think that they
can handle emailed files as well.
They also have a really cool printer that makes digital pictures
look like traditional film camera output.
As to color, cropping, and whatever - I'd get it as close to what
you want prior to sending the files. I don't like the black box to
get to the slide shape so I take the pictures so I can crop to slide
format. When going from digital to slides, I usually leave a tiny
bit more space around the object - I don't want the slide to look
If you have a full color processing facility nearby - I'd try to get
to know them. It's a whole lot easier to deal with people who you
know. That's why I recommend these folks - I know who they are and
they know me. But I'm a customer, not an owner, so no self interest
in the recommendation.