var img = $(".columnLeft").find("img[src='/path/to/existing/image/file.png']");
$(img).attr("src", "/path/to/new/image/file.png" );

var img = $(".podcasts").find("img[src='/EI/MS/Images//download.png']");
$(img).attr("src", "/Pics/ArtsAlive/download.png" );

This two liner is powerful.  It shows you how to find images on a page based on the source (src) and replace them with another image.

jQuery: Collect img tags based on src then replace them

jquery

JavaScript