Images are included as BinaryResources. It is normal. Bitmap constructor has a version that accepts byte array. You can get binary resource as a byte array and pass it to the Bitmap constructor.
@ Alir3za - If you wish to use the GetBitmap() function, then you will need to ensure that you are adding the resources using the Resources.resx file in the solution explorer and be sure to save the resources file after adding the files.
There is a little bit of magic that Visual Studio behind the scenes to only show functions for added file types in the Resource manager.