New to PowerShell. Unfamiliar with scripts.

New to PowerShell. Unfamiliar with scripts.

I found several answers on this site about renaming files in Powershell so
I'm sorry if this is a repeat. I've spent a good 5 hours on google and
various websites trying to figure this out. I really have tried to explain
this well.I am not a programmer just someone that is meticulous about
their filing. I have 14,000 picture sorted into files by year and month
but taken with multiple cameras and I want the file name to reflect the
date taken. For example October 16, 1998 pictures are in a folder called
1998 subfolder 10 October subfolder 19981016. I want the all the pictures
to be named 19981016_0001 19981016_0002 etc. I tried multiple suggestions
but it hasn't worked. Probably due to my lack of programmer knowledge
haha. Could someone help me out and give me dummy instructions? I can get
to the point where it list the folder I want to change but I'm unable to
actually change it. All of my pictures are .jpg. I tried to attach a
screenshot of what I did but unfortunately I can't since I'm too new here.
Thanks in advance!
I created a temp file of copies in case I messed it up. I started by
typing cd "C:\Documents and Settings\Brooke LastName\Desktop\Temp" then
after successfully getting my file to load I used a formula I found on
this forum.
ls *jpg | Foreach {$i=1} {Rename-Item _ -NewName
("$($.19981016){0:00000000#} .jpg" -f $i++) -whatif}
The error I got said
Unexpected token ' .19981016' in expression or statement.
At line:1 char:12 + $.19981016 <<<<
The error repeated several times
I found several formulas on the web but most created files that would
number with parenthesis for example vacation (1).jpg I want a four digit
counter after an underscore at the end of my date. ie 19981016_0001