You can re-arrange your images quickly and easily by dragging and dropping them where you need. To drag and drop an item, click on the item you'd like to move and hold down the mouse button while you move your mouse to drop your item where you want it.
You can even select multiple images and drag all of them to your desired location. Simply click and drag over all the images you wish to select to highlight them, then drag them.
If you wish to move images from one page to another, (example: from page 2 of a gallery to page 1) drag the image(s) up above the box containing the thumbnails and drop on the desired page number.