you should have thumbnails avail for the "usual" pages (knowing, its a "view" only and linking to the large full blown images only upon clicking the image or a download button.
an automatic solution to reduce the size would request the following criteria to be somehow decidable by the server process:
does the user WANT to have the large variant?
what's the local name of a reduced size image?
Since there usualy is no way for the server to answer them, there needs to be something answering them. the answer can be made only upon available infos e.g.
what browser requests it
link provides direct access to the large version
etc.
there are tools avail to generate both at once: thumbnailing the images to HAVE a smaller version and also generating "view" pages linking to the generated thumbnail as well as linking to the full detailed version of them