The blog linked to your WordPress.com username is a Tumblr blog. We can't provide specific instrsuctions until you post an active link starting with http:// to the free hosted WordPress.com blog in question. Better still, link your WordPress.com blog to your WordPress.com username please.
Dashboard -> Users -> Personal Settings -> then scroll down to Account Details. Then scroll down to where it says ‘Website‘ select the URL for your primary wordpress.com blog and then save changes.
We have come full circle. Use a text widget with a header image photo credit link to the source of the image ie. freedigitalphotos.net
<a title="freedigitalphotos.net" href="http://freedigitalphotos.net" target="_blank">Header photo image credit</a>