I don't have much time to write out a great example, but it will require something along these lines. This code is pretty rough and not optimized best practice. This is just intended to give you some idea what would be needed to get the url string for an image.
Assuming you have a collection of data from Sitefinity that includes image links...
foreach (var item in collection)
// Get a link to an image in a collection
// Replace "YourImageField" with whatever field stores the images
var links = (ContentLink)item.GetValue("YourImageField");
// Only getting the first image...
ContentLink imageLink = links.FirstOrDefault();
// Take the content link above and retrieve the image from the library
LibrariesManager libManager = LibrariesManager.GetManager();
var image = libManager.GetImage(img.ChildItemId);
string ImageUrl = image.Url;
I can provide a better example later if you provide some more specifics regarding your need.