De acordo com man rename
:
SYNOPSIS
rename [ -v ] [ -n ] [ -f ] perlexpr [ files ]
DESCRIPTION
"rename" renames the filenames supplied according to the rule specified as the first argument. The perlexpr argument is a Perl expression which is expected to modify the $_ string in Perl for at least some of the filenames specified. If a given filename is not modified by the expression, it will not be renamed. If no filenames are given on the command line, filenames will be read via standard input.
O primeiro argumento é, portanto, uma expressão regular. Você precisa alterar o primeiro argumento para um regex que corresponda a qualquer string e substituí-lo por: $(echo $(uuidgen) | cut -c1-8).jpg
find . -name '*.jpg' -exec rename 's/.*/$(echo $(uuidgen) | cut -c1-8).jpg/' '{}' \;