Your pictures used to open in picture manager, but now you are using the posted instructions to get them to open in the same program (picture manager)? If so, these instructions were not for you - you were apparently trying to do something else.

"my pictures used to open in a particular folder"
This was probably the default picture folder for Vista - "My Pictures", or whatever.

"but now I have to choose the folder"

Do you mean that you have to choose the folder to open if you double-clicked on a picture file?

If you are opening picture manager by itself, chances are you may have to choose a folder, then a file to open.