I posted a short bit on email@example.com group a while back that I felt was relevant to the articles on converting pictures of pets and people to pattern designs.
This bit focuses on taking a photo with design in mind. There is a bit of repetion of things from the previous posts -- those aspects are just really too important to not mention again.
After about 20-25 colours, you just have too many. Rather than make the pattern bigger - make the image bigger and cropout everything, but the face.
I get decent results with 4.5 by 3.5 inches. You can go bigger and still be within a decent range, but this range works well.
Make sure the photo of the person is well lit - especially on the persons face. If you have a draw program, make a copy of the photo and change it in the draw program, so that all the back ground is one colour --preferably a colour very different from their hair (ie avoid black, if the person has anything but blonde or red hair.) White is always a safe choice -- you can go back and flood fill it later with whatever you want.
If you take the photo of the person with beading it in mind -- make sure they are wearing something solid -- it will read better. Make sure they are well lit on the face. Make sure they aren't blended with the back ground. (green grass usually looks rather nice when the person is wearing a bright primary colour.
Another important part of taking the photo -- take as large a photo size as your camera/memory card will allow. This will give you more pixels per sq. inch and better resolution when you enlarge the photo. This is not only good for making better patterns, but when you take that perfect photo, you want to be able to blow it up and hang it on the living room wall.
If your original photo size is only 4x6, then when it gets enlarged to 8 by 10, you will have blurry areas with a grainy look overall. Treat every photo as if it will be the one you plan on enlarging for your wall, when you take it.
Another technique is to print the photo full size on regular paper(8x10) on a very light setting (draft). You really don't want the colours to be too strong for this technique. Then with a sharpie "trace" color blocks that jump out at you from the print. Rescan this in and use the modified image as your guide when you assimilate it. It will be easier to edit.I find this works especially well when I use the image as a guide and sketch over it in the design program. (Beadtool will allow you to place the grid over the image and "trace" over the image as you go.- This can be really usuful with this technique.)
If you used the assimilate on your program, and need to get rid of the "outlines" Just use the flood fill and the next closest colour sitting next to that section.
Always look at the pattern as a thumb nail image -- this will give you a better feel for how it will read to others viewing the pattern. Looking at the image too close up will just look like a bunch of dots. Give it some space, and the dots organize into a pic.