You have defined:
src.UriSource = new Uri(textBox4.Text.Trim(), UriKind.Relative);
Based on that, the filepath formed is pointing to your Bin/Debug folder.
Based on the error, it looks like the image is not placed in Bin/Debug/ folder. You need to make sure that the path is correct.
Have a defined image folder where you keep the images. If inserted, place the image there. At the time of picking it up for showing/editing, use the folder path (fixed and known by you) along with the filename to display.