It's like Windows Picture Viewer or Zeed but it has more options and autosizes pictures to the size of the window while maintaining proportions. Ex: I pull the window wider, it automatically also makes the window taller to keep proportions. It's freeware but there are..."workarounds."
Artsee is similar but It doesn't have the functionality I want. it doesn't auto zoom when you resize the window and the window doesn't automaintain its proportions.