First, you'll have to reduce the size of your images to below 2Mg. You can either reduce the definition to 72 dpi (native screen resolution) and/or reduce the size to 1200x1200 pixels.
You go in the gallery module by clicking on Gallery in the yellow bar of the menu, then click on "my images" or new gallery (don't really remember) and from then on, menu boxes appear to make you upload your images.
I actually haven't posted to my gallery for a little while, so I'm a little sorry if my explanations are not quite "right on".
Give it a try and let's see if you can work it out.