Libreoffice/Openoffice Calc - append string to cells

This is just another option,

just click the function wizard and select concatenate, in that enter which column you want to enter as text1 as and second column in text2. Then when you click OK you will get an concatenated column like below image

so in the C column you will get as a1.jpg.

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.