delete files matching pattern

A string contains “a number followed by an x followed by a number” if and only if it contains a digit followed by an x followed by a digit, i.e. if it contains a substring matching the pattern [0-9]x[0-9]. So you're looking to remove the files whose name matches the pattern *[0-9]x[0-9]*[0-9]x[0-9]*.jpg.

find /path/to/directory -type f -name '*[0-9]x[0-9]*[0-9]x[0-9]*.jpg' -delete

If your find doesn't have -delete, call rm to delete the files.

find /path/to/directory -type f -name '*[0-9]x[0-9]*[0-9]x[0-9]*.jpg' -exec rm {} +