Basic syntax for command-line resizing:

convert -geometry 248x186 input.file output.file

Some basic sizes
Landscape thumb: 248x186
Smaller Landscape: 194x145
Portrait thumb: 133x178


A Basic script

#!/bin/sh
for f in $* ;do
 convert -geometry 80x80 $f t_$f
 echo "<a href=\"$f\"><img src=\"t_$f\" width=\"80\" height=\"80\"></a>"
done
# end of script