It sounds like you're rendering a preview, not the image.
If you already rendered your image, push F3 to save it as originally suggested in Gandalf's answer.
However, if you want to render it and get a file without having to touch it later, follow these steps:
I suggest you first set your output directory to your desktop or another non-temporary place:
Set your animation length to 1 frame long, like this:
Now make sure you have the proper image file output format selected, like PNG:
Now hit CtrlF12 to render the "animation" (which is actually the one frame) or click Render > Render Animation:
After your single frame finishes rendering, you will have the image on your desktop or wherever you set the output.