Above is an attached 38kb ".jpg" file uploaded using the "Upload Attachment" tab at the bottom of the forum post editing window, and then posted using the "Place inline" button just below the editing window once the file has been uploaded to the server using the mentioned "Upload Attachment" tab. There should be no difference whether the file is posted from a pc or a mac.
I'm not familiar with a ".tif" extension, but if you're referring to the ".tiff" format (Tagged Image File Format) extension, those are not allowed for technical reasons.
Allowed formats on the forums are ".jpg" ".jpeg" ".png" and ".gif". File-size limit of 50kb (50,000 bytes).
The first thing would be to "check your work" and eliminate the possibility of user-error as the most common reason. I don't have a mac to verify image uploading to the forums so if any of our members use a mac that would be helpful info just to verify one way or the other, tho' I can think of no reason why that should be a factor.
Also, which browser are you using and which version is it?