There are two different ways you can rearrange the order of your images within an image gallery.
Option 1: Within the Content tool
While viewing the image gallery content block, you can reorder images by clicking and dragging on the Hamburger Icon (appears as three lines stacked on top of each other).
Option 2: Within the Design tool
While viewing the website preview, simply click and hold on an image to grab it, and then move it where you'd like to within the gallery. Be sure to click and hold on an image, similarly to how you would move the images from the option above.