This can happen if you've mis-named the file (specifically, if you have given it the wrong file extension).
For example, if you were to save a file called xyzzy.jpg but accidentally forget to use the Save As Type field to change it to JPEG, it would have PSD-formatting internally but have a .jpg extension, and Photoshop would subsequently emit the message you cited upon an attempt to reopen it.
If this is what's happened, you'll need to figure out what the real file type is, and rename the file so that it has the appropriate extension.
I don't know what software is available for Mac to do this, but if you have a Windows system there's a good viewer called Irfan View that will determine the type by looking inside the file and give you the opportunity to rename it properly.
Sometimes, the files especially the .PNGs were being saved from different OS or different application.
This is not the caused of a virus or OS problem like most people assumed.
I went through this experienced and I solved it by
#1. Open the photo from (MS Paint) or something similiar application if you're using MAC
#2. Save it as .PNG too or desired file extension with different name.
#3. ... Now, open it with your Photoshop... you can now also "Place" the photo on your canvass.
Hope this helps.