If you want to change how a drawing shape looks, you can easily use the border and fill shapes. However, if the object is a bitmap, like a digital camera photo or a JPG you got off the internet, you need to do something different.
You don't have the capabilities of GIMP or Photoshop but you do have quite a bit to work with. Here are some examples. You just click on any JPG or other shape, and you'll see the Pictures toolbar. Click on the rainbox icon on the Pictures toolbar and you'll get the other toolbar. Change the brightness/contrast (the sun), the contrast, and the Red Blue and Green values. The range of possible values is 100% to -100%, so higher for more intense and less for less.
Use the tooltips to determine what each icon does. Move your mouse over each icon and you'll see a description.