OS X Numbers. Column labels to numbers
Generally, what is done is to create a Column Header.  You already have a perfect example of this in the table shown in your question. Row 1 contains field names for each data record.  i.e.: "Image", "whale_00", etc. (See "Work with Rows and Columns" in Numbers Help)


Select Row 1 and convert to Header Row




If you wish, you can create a header row with you own number system. This would be much faster and less tedious than renaming the column designations.  (In answer to your question: Cannot presently be done in Numbers.) NOTE: By design, Numbers will use your Header Row Data in calculations so you can easily identify the data being referenced.


Add a Row Above your first row.
Enter ordinal numbers in the first and second cells.  For example: A1 = 1, B1 = 2
Select both cells in the row and drag the "yellow dot" across the width of the row to number each column.
Convert this to a Header Row