Don’t use the ‘create gallery’ option. Click on ‘upload files’. From here you can select (or drag and drop) the files you want to upload (1 or multiple). The images will appear in the media library. Make sure each image is ticked (and add titles etc). Select medium size for each image. When ready, click on ‘insert into post’ (bottom right of screen). See the link above (31 July post) for more info. Hope this helps.