Next we apply the Motion Blur effect to the area of the image that we've selected. You'll find this in the Filters menu under Blur.
Clicking on Motion Blur in the menu brings up a dialogue box where we can set the parameters of the blur effect. GIMP does not provide full-screen previews on the actual image, so check the Preview box and drag the image in the preview window until some recognisable part of the image is visible.
Next, check the button for Linear blur, and set the Angle slider to 180 degrees, so that the blur is horizontal and left to right. As for the length slider, that is up to you depending on the extent of the blur you wish to create, but for the supplied starter image a value between 60 and 90 pixels will produce a strong effect without completely overdoing it. Click OK when you're happy with it to apply the effect to the image. You'll see a progress bar along the bottom of the screen as the program renders the effect.
When the effect is applied, cancel the selection by using the keyboard shortcut Shift+CTRL+A, or by choosing None in the Select menu.
The GIMP is quite fussy about how it saves, so we'll need to flatten the image before saving. There is no default keyboard shortcut for this function, so go into the Image menu and select Flatten Image.
You can then save the finished image as JPEG file using Save As in the File menu, but be sure to save it as a different name to the original, or you'll over-write it.