Do you need to do this once or is this going to be a repeated task every week/month?
If it is something you need to do just once, here is what you can do:
Next (right) to the column where your numbers are open (insert) a new column.
Assuming the following: Numbers are in column A, New column is column B.
In this new column B in the top cell (B1) write:
=A1&“.jpg”
Now copy B1 all the way down to the end. In B1 type [Ctrl]+c then Hold [Shift] and hit [PgDn] until the end then press [Ctrl]+v.
Highlight Column B, [Ctrl]+c, then [Edit] [Paste Special] values only (No formula’s) this freezes the calculated data.