How can I get text on a circle and use a background image in the text?

[Ofnuts]

Two cases:

You don't warp the underlying image:

• The tutorials you have show you how to obtain  a the curved text as a "path".
  
• Instead of bucket-filling a selection from it, use the selection to create a hole in your layer, showing the image in a layer below
  
• Or use the selection to copy-paste from another layer
  

You want to warp the underlying image

• You can use the polar coordinates filter to wrap an image in a circle (in you case, you want your flag to be a part of the circle, and so will have to add margins on the sides
  
• However this stretches the image and also blurs it a lot, so your starts won't look like stars and will be blurry.
  
• A possible workaround is to find a font with a character that draws an American flag, and to warp it the same way, and then paint the resulting warped flag before you use it as above.