It sounds like you may be using the "Import Image Sequence" option when you actually only want to bring in one image.
When you import the image, make sure that IMAGE SEQUENCE is turned off.
(After Effects tries to import the entire list of numbered frames (img_01, img_02, img_03...) if Sequence is turned on)
here's what it looks like on a mac or pc
Here's the long answer (help file link):
I believe that was the problem, however now when I open After Effects I get many pop-ups saying certain plug-ins are missing, and I am now unable to import ANY files to my project.
the plugin missing might be a codec.
if the footage isn't supported on your OS, it's possible that the problem is in the available codecs.
on my PC, I can't import Apple ProRes footage. (Need a mac)
on my Mac, its been a long time since I attempted to open Windows Media footage. (Need PC)
BUt they import if I'm on the correct OS without any problems