To view a jpeg image in WMP, right click the image and select Open With then Other Programs and scroll to WMP. DON'T tick the box which says Always Use this program for this type of file.
If the WMP doesn't come up with the image, then it would appear WMP is not configured correctly. Maybe your File Extensions have been changed inadvertantly.
These can be found in:
Start > Settings > Control Panel > Folder Options > File Types.
(I am assuming you have XP - I am quoting for XP).
Make sure the type of file you are trying to read in WMP has WMP as the opening program.