Ok... Let's start with your Settings. on the left hand sidebar in The Administration Panels - click on "Settings" -> "Media". This section allows you to control what size WordPress will make your "smaller" images when you upload them. NOTE: WordPress will not create these auxiliary images if the original image that you upload is smaller than any of these sizes.
In the default settings, you will see that the "Large size" is set to 1024. Unless you upload an image larger than this, you will never have a "Large size" image.
Once you have your settings set the way you want you can create a new post - to upload the images, click on the little rectangle to the right of the words "Upload/Insert" - this is located directly under the title of your post.
This will bring up a "thickbox" dialog with three tabs along the top, the first tab will give you a button to select files from your computer to upload. do this and when all are uploaded, click on the "Gallery" tab. Here you will be able to click on "show" for any image and there are many options to send an individual image to the post box on the "Edit Post" screen. You also have the option to send all images to the post box using the gallery shortcode.
Hope this gives you some kinda insight into this!