After downloading a website, I decided to test how well the website could be
viewed offline by first going offline / disconnecting from Internet, then
deleting cookies/cache from browser. No files were moved around.

Then via file:///, upon opening the webpage that should have visible thumbnail
images, the thumbnails are missing. The webpage uses a lightbox script to
"balloon" thumbnail images. If I click on the location where the thumbnail
images should be, their respective larger images will show up (after
"ballooning"). Why aren't the thumbnail images showing up like they are
supposed to?
To debug this in Firefox, I used Inspector to examine the element (one of the
thumbnails) and compared to the original webpage. The original website goes
like <img src=http://blahblah/img-300x224.png with absolute URL replacing
the blahblah. On the archived/offline WinHTTrack copy, a relative URL is given
instead with <img src="../wp-content/uploads/img-300x224.png"> and when the
cursor hovers over that URL, it will display "Could not load the image"

So I looked at the offline URL, then looked at the archived WinHTTrack
directory of the website and went down a directory as per the ../ then went up
two directories as per wp-content/uploads/ and the thumbnail image is there.
It works fine when I open it from the directory. So why is the webpage opened
offline not displaying the thumbnail image even when relative path is correct?