Red X over thumbnails means what?

[dionj]

When I try to view a directory using pane style for  thumbnails, I frequently, but not always find the thumbnails shown very tiny (approx 6mm square) inside a proper thumbnail box, with a big red "X" or cross over the tiny picture.  

If I double click on the window of such a thumbnail (of a .JPG file) then it brings up Irfanview and shows the file/picture normally.  But the thumbnail remains hidden with a red X.  

What am I doing wrong?  Why are my thumbnails not being shown?  SOME of them are being shown properly, but most get the red X. Note all the files a JPG pictures and are accessible and viewable otherwise.

Thanks Nikos, it's a terrific window manager.  I am still learning new things about it after all these years. -Dion

[nikos]

check your thumbnail extraction byte size limit in Tools | Options | General, it may need increasing