The Linux Flashtool depends on a number of libraries of which most are available on 'modern' Linux systems, with the notable exception of libjpeg.so.8, resulting in a message like this:
./flash_tool: error while loading shared libraries: libjpeg.so.8: cannot open shared object file: No such file or directory
Attached to this post is a x86_64 version of libjpeg.so.8 I built as a workaround. Download, unzip and put in the FlashToolLinux/lib directory.