Open the folder containing the images you want to upload but resize it so it doesn't cover the whole of your screen, you need to be able to see the box on the page below you are going to drag-and-drop into. Select* the images you wish to upload then click on one of them and, keeping the mouse button pressed, drag it to the box on the webpage. Then click ["Start Upload"].
* If you want to upload all images in the folder Ctrl+a will select them all. To select a group of adjacent images click the first and, while holding down the Shift key, click the last. To select by individual image highlight each by clicking on it while holding down the Ctrl key.
NOTE: there is a limit of 100 files in any one upload and each file must be smaller than 8MB.