For future reference, there's a lot of useful information within the ArtRage 2 manual (found under the "Help" menu in ArtRage), which can help answer this and similar questions.
With ArtRage open and your image on screen, click on "File" and select "Export as Image". In the window that opens, type in a file name for your image and then select the type you wish to save your image as in the "Save as Type" box. Click "Save" and you're done, but be aware that if you save your image as any other type besides the ".PTG" you will loose any ArtRage specific information for the file. This means if you don't save it as a PTG file and wish to go back to do something to it later, you will have lost any layers you had, as well as any specific mixed media (chalk, paint, layer texture) you may have had. In some cases (such as with a .jpg extension) the image will also be compressed to some degree and you will loose some detail.
Nothing is easy to the unwilling.