When creating a post or page on your WordPress.com blog, you have two editing modes available to you Visual and Text (HTML), depending on which you are most comfortable with.
Please note that the Text editing mode is only available when using the post editor in your site’s dashboard and not via the WordPress.com home page quick post editor.
Switching between the Visual and Text editing modes is easy. Just click either the Visual or Text tabs directly above the right side of the editing area. See here > http://en.support.wordpress.com/editors/#text-editor
Here are the basics:
The accepted file types are here http://en.support.wordpress.com/accepted-filetypes/#images
This is a link to instructions for uploading a header image http://en.support.wordpress.com/themes/custom-header-image/#uploading-a-custom-header-image
For uploading images posts and pages see here http://en.support.wordpress.com/images/
Here are more details about the options for uploading images http://wpbtips.wordpress.com/2009/04/26/images-the-three-link-options/
For editing see here http://en.support.wordpress.com/images/image-editing/
For troubleshooting images see here > http://en.support.wordpress.com/images/troubleshooting-images/
You can simply focus on uploading the images as is, rather than focusing on minor details like removing borders, which will require fiddling with coding. But if you still want to remove image borders then Staff will help you. Go to the sidebar on this thread and type in modlook for Staff attention. Then subscribe to the thread so you are notified when they respond and please be patient while waiting.