(this is against 0.9.0) A debian user reported that thunar wasn't refreshing the thumbnail when an image had been changed (in this case, using jpegtran, but convert works too). The mtime has been changed so according to thumbnails specs [0], the thumbnail should be regenerated. [0]: http://people.freedesktop.org/~vuntz/thumbnail-spec-cache/modifications.html
Ok, sorry for the noise. Problem was that those jpeg contained embbeded thumbnails, wich were'nt refreshed. Cheers