Inserting Text within a filename

I'm brand new here, and I hope this is the right place to post this. I have a spreadsheet of filename paths. For example:

assets/images/psa/testimage.jpg

I need to add "_thumb" (without the quotes) into that filename so that it reads:

assets/images/psa/testimage_thumb.jpg

So basically, I need the "_thumb" to go after the file name and before the extension.

Can anyone tell me how to do this. I've tried playing around with the CONCATENATION function but can't seem to figure it out.

Thanks in advance for any assistance! I would hate to have to add _thumb to 600 filenames

with link in A1
=SUBSTITUTE(a1,".jpg","_thumb.jpg")
or
just
=SUBSTITUTE(a1,".","_thumb.")

THANK YOU!!!!! I can't believe how quick the response was and how easy. I knew there had to be a way to do it with Excel...but it was just way beyond my capabilities.

Have a great day/night/whatever!