@MarkONeill: Thanks for this additional information. On the Flickr authorization web page, did you choose the second (right side) option (see image at http://i.imgur.com/EmrHy.jpg
)? I just tested it using that option, and after I then authorized the Sizester app, Flickr said I could now close the authorization browser page, and after I did that, I checked to see if the image from Sizester had uploaded to my Flickr account and I found that indeed it had (under "Your Photostream"). Please try that process and confirm it works. If so, maybe what we need to do is try to implement a success message in Sizester to provide better confirmation of the process having completed successfully. As for your other point about asking for authorization again, the reason is because we believe it would be improper for Sizester to store user login credentials since there's really no highly secure way to do that.
@Meikah: There's a free 15 day trial (full functioning), and after that there's a one time license fee of $10 (precisely $9.95) for a single PC (and there are license packages for multiple PCs that are less than $9.95 per PC).