This should do:
"To whom it may concern:
I confirm that I am the artist who produced the sketch illustration of St Maud's Church, which is now in the possession of the purchaser, Mrs Ethel Higgins.
I further confirm that I retain the copyright in the said work.
I hereby grant Mrs Ethel Higgins the right to have a canvas reproduction of the sketch made by Asda, and indemnify Asda against any claim for copyright infringement which may be made in respect of such reproduction.
Signed: Janet Pearson, 02/08/11"