I'm using Drupal 4.3.2, with the 4.3 Image module.
I discoverd this trick, and I thought I would pass it on for the other newbies (like me.) If you have the image module settings set to display the LAST thumb first, and the newest->oldest images first, within the catagory, sometimes you might find that you are able to load 2 or three images per minute. That can be a problem if you need the images to be in a particular order because the timer is set for minutes, rather than seconds. What that means is, if you load more than one image per minute, they might not show up in the correct order because they all have the same time stamp.
Not to worry. Load all the imgaes for your particular gallery or sub-gallery. Once you are finished, go back and check to see if they are in the correct order. If not, you can manipulate the image display order by changning (editing) the time stamp. You can use the time setting to move the images around to the order that you want. Also, if at the end, there is one particular image that you want to be the main thumb, edit the time stamp of that image to make it the appear to be the last (latest) image loaded.
For example, I have five main catagories, and each catagory has multiple sub-catagories. Each main catagory shows a thumb (the last image loaded) as it's sample (or main) image. I chose one image as the main image for the catagory, and edited it's time stamp to make it the last image loaded. I used the same trick for each of the sub catagories. Now, all the images are in the correct order, and each catagory is showing the best main thumb as it's catagory sample image.