> Is there an easy way in GIMP to make corrections to those distortions
> that happen for instance, when we try to take a photo from a very
> high building (when the straigh edges get a bit rounded or inclined).
Yes, there is. However, you'll have to wait for 2.4 version (or
install development version). Filters > Distorts > Lens Distort.
Alternatively, you can try Gimp Wideangle filter:
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