Adding an image to the gallery these days is pretty straightforward.
Go to the gallery sectopm, then click the "upload" link in the little strip below the thumbnails that are displayed.
In the resulting form, be sure to choose an appropriate category and enter a title and brief description for your image. Then click upload.
If you are experiencing problems, things to check are the size (dimensions and file size) of the image you are trying to add. There is presently a 500KiB limit on uploaded files.
