Look at your code:
imgInput = new Image<Bgr, byte>(ofd.FileName);
pictureBox1.Image = imgInput.Bitmap;
imgInput is an Image instance, and Bitmap is a separate class that derives from Image - that doesn't mean that the Image class contains a property which is the derived class!
Think about it: an apple is a fruit, but that doesn't mean every fruit is an apple - an orange is a completely different fruit and can't be treated in the same way.
Images and Bitmaps share the same relationship - not every Image is a Bitmap, but every Bitmap is an Image!
And if you look at the documentation for the PictureBox class, you will see that the PicturBox.Image property accepts an Image anyway - so any class derived from Image goes straight in. So chnage you code to
imgInput = new Image<Bgr, byte>(ofd.FileName);
pictureBox1.Image = imgInput;
And it'll compile.
You should expect to get syntax errors every day, probably many times a day while you are coding - we all do regardless of how much experience we have! Sometimes, we misspell a variable, or a keyword; sometimes we forget to close a string or a code block. Sometimes the cat walks over your keyboard and types something really weird. Sometimes we just forget how many parameters a method call needs.
We all make mistakes.
And because we all do it, we all have to fix syntax errors - and it's a lot quicker to learn how and fix them yourself than to wait for someone else to fix them for you! So invest a little time in learning how to read error messages, and how to interpret your code as written in the light of what the compiler is telling you is wrong - it really is trying to be helpful!
So read this:
How to Write Code to Solve a Problem, A Beginner's Guide Part 2: Syntax Errors[
^] - it should help you next time you get a compilation error!