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I see the same thing... except I am using v0.13 and I used
the Tools menu "Remove Old Thumbnails" which apparently did
nothing.

What I was really trying to do was increase the display size
for thumbnails... but after having set it in the
preferences, neither the step above norchoosing "Delete
saved thumbnails" in the browser tab seemed to do anything.

As a work around I manualy removed the .thumbnails directory:

# cd pictureDirectory
# rm -rf .thumbnails

after reloading the directory with gthumb it recreated the
thumbnails just fine... at the new size I had set for them
in the preferences.