@nigel actual import time for 20,000 products is about 8 hours or more—depending on the speed of your server.
For your other questions you may be spending some time in the dataflow forum.
OK—I finally got everything to work last night. Here is what I did:
1) Added a / infront of the image name for ALL THREE images (base, small, thumb).
2) Used the store name default
3) Resized all of my images 265x265 (base) 135x135 (small) 75x75 (thumb)
Resizing the images took care of the php code errors I was getting. I was trying to use my base image for all three, but that is aparently not a good idea. You need to use a program like FastStone Photo Resizer to resize all of the small and thumb images before you import them. When you import, all three types of images should be in the import folder.
Also—incase any of you haven’t figured this out yet --- to get the correct excel format for importing, add one or two items to your store and then export everything. That will give you the correct format to use to import.