They are the victim of automatic resizing by forum software.
Upload files that are slightly larger than required, say 1600 x 1200 pix, and they should look fine. Linked images might be impacted by the site delivering them.
Images with bright colours and in colour spaces not sRGB can become flat and dull on web pages. Thus forcing the image to sRGB before uploading can be helpful.
I looked into one of the files and the pixel size was 1050 x 1400. Thus the image *should* fill a normal web page. Perhaps dropping the dpi setting or use "save for Web" will help? The image here is set to 300 dpi, it was 72 originally.
As to being "critically sharp", that is always tricky to ascertain on small-sized web images. I added a touch of sharpening to make the image display more advantageous. This is a matter of taste more than an exact science for web pages (entirely different from pre-press and printing work).
