This was the first program I wrote with C# / .NET. If I had to do it over again, I'd probably do some things differently. But, it at least shows how some things work.
What I wanted was a program that would take a directory full of images and create an HTML page with thumbnails and links to the full images. I also realized that I didn't really want the original image posted in most cases -- our digital camera takes photos that are 2048x1536 pixels -- much bigger than I normally want to post on a web site. So I included an option to limit the size of both the "full-size" image as well as the thumbnail.
You can also select whether or not to use JPG or PNG as the output and, if JPG, the quality level.
The program uses the .NET classes for images extensively. Opening, resizing, and saving an image in a different format is incredibly easy.
You can download the source code and the built program here.