The solution provided in Easily Renaming Multiple Files works perfectly well if we want to add a prefix to a file. This solution does not work too well if you want to add a suffix.

Objective: For all jpegs in a folder add the suffix _tn after the file name and before the extension.
Example: file1.jpg = file1_tn.jpg

In the console type the following:

for i in *.jpg ; do mv $i `echo $i | sed 's/\.jpg/\_tn.jpg/'` ; done

If the file name was in CAPS and and you’d rather have your file name in lowercase pipe the following on:

for i in *.jpg ; do mv $i `echo $i | sed 's/\.jpg/\_tn.jpg/' | tr [:upper:] [:lower:]` ; done