I think he/she is talking about correcting the lens distortion; Is there some kind of basic "skew" feature in PhotoPad?
Until then, I suppose OP could try:
Free-form selecting the building
Exporting that free-form building into Paint - Microsoft Paint (preinstalled with Windows) comes with a "skew" feature in its Resize menu
Save it, then remove the background with PhotoPad's builtin feature for that
Insert that building again