You can change the order of your images after uploading them. This will also affect the way they appear on social networks. You can see in the previews the effect on the display. To re-order the images in the Central panel, simply drag and drop the image thumbnails to the location you want in the order.

To re-order images in the Social previews, first active edit mode by selecting the edit button in the top right corner. Then, same as in the central panel, drag and drop the thumbnails to change their order: