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.
|IMG_8168ABCD by citori525|
|McKinley meadow by TimR32225|
from Natural meadows
|Flare-well to a Classic Flying Machine by cjf2|
from Flying Machines
|_DSC2146 by jerste|
from Helios-44 II
|Leopoldsteinersee by RaCor|
from Landscape - Colour #3