Does dpreview gallery recompress photos?
I did the test you suggested and recompression is evident, they leave unchanged the pixel size though.
Pity, because the slideshow is nice, but recompression is so strong I could notice it even with no zoom, then zooming, and even more comparing original vs full size dowload, revealed the problem.
|Valley by the light of a blue moon by cjf2|
from Down in the Valley
|Lake Erie Stone Pier by yobbyt|
from Dock or Pier
|Hong Kong by Zephyr1977|