Create a new folder in PCManFM : right click in the middle of right part, first menu : New → Directory.
Get the thumbnails : display your images in any image viewer, go to the .thumbnails directory, browse there and see if the thumbnails stocked there fit your needs.
If not, you can use "convert" from the ImageMagick suite, with a
script such as:
for i in `ls *.*`;do convert $i -resize 48x48 modif/$i ; done
You change the value "48x48" with what suits you. This is in pixels.
I just figured out how to get this to work in thunar. I had to install thunar along with a few plug-ins.
Which ones ?