Why is this so difficult? I just want to add one single image into a post that already has a media gallery on it. When I try to upload a new file into the media library from my post, my image shows up both in my post and in the gallery – but I don’t want it in the gallery!

Just found this at Not the Official WordPress.com Gallery FAQ:

Any image you upload in the same Post or Page where your Gallery appears will automatically be included in that Gallery. Inserting an image which was uploaded via “Media>Add New” in the Dashboard is not attached to a specific Post or Page and will become attached to the Post or Page where your Gallery appears. This causes the image to be displayed twice, once in the Gallery and again indivdually. The solution is to upload the image to your Media Library from a different, existing Post or Page. Once your images have finished uploading, save your changes and then close the Image Uploader, then go back to the Post or Page where you want to insert the separate image. Click on the “Add Image” button and in the Image Uploader, click on “Insert in Post” button to insert the individual image in your Post or Page. In order to avoid navigation problems, you should link the image to either “None” or “Image URL.”

Does that not seem, well, ridiculous?