It didn't work.
Oh you.
Two ways:
1) In the Toolbox, there is a tool called "Select by color tool". It looks like a finger pointing at the red box in a stack of blue, red and green boxes, on the top row. Choose that tool, then click on the white space. Then press delete on your keyboard. THEN do Image > Autocrop.
(This will "select" all the white around the image, and pressing delete will delete all the white.)
2) Use the rectangle selection tool, also on the top row. Put a rectangle outline around your image. Go to Select > Invert, then press delete on your keyboard. Then do Image > Autocrop.
(The rectangle selection tool will "select" just the image. Doing Select > >Invert reverses it, so that everything BUT the image is selected. Pressing delete will then delete everything but the image. If you press delete without inverting first, it will delete just the image.)
Okay, three ways:
3) Use the Crop Tool (looks like a scalpel or cutter), it works the same as the Rectangle Select tool except that everything outside the box will be dark, so you know what you're deleting. After you put a square around your image, click the mouse in the middle or press enter to delete all the dark outer stuff. Then Image > Autocrop.