If you take a picture of a building, more often than not, you are at street level. To get the picture content that you want, you have to use a short focal length or wide angle lens, or maybe you are using a phone camera which will give the same problem, the building appears to lean, it has serious foreshortening.
This can be corrected using GIMP editor
Open the photo in GIMP and select the PERSPECTIVE tool from the tools menu
Grab the top left corner of the layer and drag to the left until the buildings edge is straight.
Do the same for the right side.
You may find that this changes the left hand side of the image slightly , so if necessary go back to the left and straighten again until the whole image looks right
Lastly click TRANSFORM and the image correction is complete
The final corrected Image with straightened sides
You can now go to the file menu and EXPORT with the filetype you need for your page. PNG is the usual favorite filetype but Jpeg can be useful where minimum filesize is important.
Another frequently used application for the transform tool is for book titles on a perspective surface, check out my tutorial for Book Covers