Retrieve SharePoint List items from a picture library and display them in a sharepoint webpart created using visual studio

you can use SPPictureLibrary and Random number classes in C# webpart code-

SPWeb web = SPContext.Current.Web;

SPPictureLibrary picLib = (SPPictureLibrary)web.Lists["Photos"];
int count = picLib.ItemCount;
Random rNumber = new Random();
int index = rNumber.Next(count);
string sourceurl = web.Url + "/" + picLib.Items[index].Url;

var image = new Image();
image.ImageUrl = sourceurl;
image.Height = 300;
this.Controls.Add(image);

Please check this for how to create a webpart - Walkthrough: Creating a Web Part for SharePoint

You can also use Picture Library Slide Show webpart - http://www.splessons.com/lesson/sharepoint-picture-library-slideshow-web-part/