Opening a New/Blank Image & Image

File>New brings up this

Advanced options for different background ie transparency
In advanced you can also change the resolution of a new image.

 Templates

To open a new blank canvas or start a new project from scratch you need to go to File > New and open one of the templates (there are many more to choose from now) or make one of your own size.

To work on an image you can go to file > open > browse through your chosen folder until you come to the picture you want then click on open - It will then be open in your window.

*Ifyou want to open a RAW in Gimp you will need to download either RawTherapee or Darktable.
Browse to the picture you want from Gimp and the RAW program will open the picture and when you close that program the picture will open in Gimp.