A very used tool in gimp is the perspective tool. Here in four steps you can make an image look much better. I am using this tool for many architectural photographs.
1.Load your image that need perspective correction and click on perspective tool.
2.Click and drag from a corner of the image
3.Repeat the process for every corner as many time is necessary-i use as guide the layer margin(yellow black line) but from the ruler you can drag beforehand some guide lines.
4.Clik transform from the dialog.
And just save.