I uploaded a picture to my gallery, but I cannot figure out how to select it to put it into a post. I click to add the image, and the gallery comes up, but I can see no way to choose that image and add it.
Don’t use the gallery. It is a special case thing for inserting a group of images. To insert an image that is already uploaded, from the post editor, in the “insert image” window, click on the “media library” tab at the top, find the image and click the “show” link, make any selections and changes you desire and then click the “insert into post” button.