Really sorry to hear they are using your image without your permission.
I assume you've sent them an invoice for the £26 (although this sounds a bit cheap) which they are refusing to make payment against. If this is the case, take them to the small claims court, information on how to do this here
http://www.which.co.uk/consumer-righ...-claims-court/
Out of interest, who decided that your image was worth £26?
Regards
Nick