ffmpeg -i MOV00502.flv -y -f mjpeg -ss 0.001 -vframes 1 -an MOV00502.jpg
Flashcomguru.com: Using ffmpeg to create jpg thumbnails from flv files
Install ffmpeg from mediubuntu, then:
# ffmpeg -i MOV00477.MPG -ar 44100 MOV00477123.flv
mogrify -resize 1920×1200 *.jpg
Batch Resize Images Using Imagemagick | Digital Photography Tips and Techniques