You can only do it via the link icon, then select upload. There is no preview for tif files.
http://bioimagingcore.be/q2a/?qa=blob&qa_blobid=12786994099093256657
See http://www.question2answer.org/qa/32208/why-cant-i-upload-a-pdf-document?show=32208#q32208
As far as I know the software only accepts JPG, GIF and PNG but not tif, at least for display.
May be but you are limited to a maximum size of 1 MB . From the admin interface you can increase the limited size to 2MB, otherwise you have to change the PHP file, see http://www.question2answer.org/qa/23391/maximum-size-of-uploads-8-0-mb-max-8-0