Question : Tilted angled text in GIMP or Paint Shop Pro?

If I have a picture of a highway billboard which is not taken straight on, how do I take another 2D image and angle it so that it looks like it fits the angle of the billboard?

Obviously if the billboard is just rotated but straight on then I'd just rotate the other image before pasting it into the billboard image. However if the image isn't straight on then I need to sort of crush the new image and tilt and angle it somehow.

What is that called and is it possible to do in GIMP? If it's not possible in GIMP then how do I do it in Paint Shop Pro?

Answer : Tilted angled text in GIMP or Paint Shop Pro?

Gimp lacks a free transform tool like Photoshop. You can press [shift]+[p] for the perspective tool.
Use the perspective tool on the layer containing whatever it is you want on the billboard.

See this link for more info: http://docs.gimp.org/en/gimp-tool-perspective.html
Here's a Gimp tutorial that utilizes the perspective tool: http://gimp-university.blogspot.com/2008/03/perspective-and-layers.html