It's to do with the case of the letters in the link. The return page is "SlopePICS.HTM". The links which don't work are pointing at "slopePICS.HTM", with a lower case s. It works on your computer because Windows is not case-sensitive for links. Your website is hosted on a server which uses Linux, which is case-sensitive. It is best to use all lower case characters on pages, folders and extensions within your website (eg "slopepics.htm"). This would prevent this problem.
Best phone camera 2017