Version:           2.5.0 (using KDE 4.7.3) 
OS:                Linux

I just observed that flickrexport reports an "unknown error" when it attempts to upload images that exceed the maximum allowed filesize. The major problem is: it first attempts to upload the file, which takes a long time depending on the internet connection, and then reports the unspecified error, which leaves the user at first helpless.
I suggest to make flickrexport to check for the file size and report right away that those images won't be accepted. Since pro users can store bigger files on flickr, and I'm not sure if the flickr API reports back if the user is a pro user, a second suggestion:
to have two fields for max file size and max video size. Then flickr export checks only against those entries. A short info in the interface can explain to the user what the current limits are, and suggest maybe the non-pro user limits at first.
flickr has currently these limits: 15 MB per image (and, I believe, 100 or 200 images) or 2 movies (each <= 150 MB)/month for non-pro users, and for pro users unlimited amount with 20 MB per image and videos up to 500 MB each.

It would be a question if the quantity limits for non-pro users can also be checked, but that would be more a question to the API than to flickrexport, even though this would be nice to have (not for me since I am a pro user, but maybe for others who are not). I am not sure if a total of 200 images is allowed throughout, or a total of uploads per month.

Reproducible: Didn't try

Steps to Reproduce:
None, this is a wish.

Actual Results:  
None, this is a wish.

Expected Results:  
No guessing what could be wrong