Hi Osso - I'm not sure about Picasa Web Albums... or are they Google Albums. I just click the upload button from Picasa and up they load... so easy, quick once you have your settings sorted... if you can manage to download it, it would be worth it for your purposes. There is a limit to the amount of GB you have - but it's a fair size so I wouldn't worry too much. You are limited in this software for the exif information that is provided for photographs - but if you are scanning then this will be no problem.
I've not used gimp... people have informed me they use it quite well but again, they have basic editing funtionality.
Photobucket, I don't like - the quality is pap to be honest. It also has a GB limit and you can't put more that 2500 in one album... it has more restrictions, to my knowledge, than picasa - I am biased tho because I really don't like the quality of the pics on PB and I do think Picasa quality is better - not brilliant - but certainly better.
Most of these free software packages change your photos to some extent, there's not too much you can do about that.
Let us know how you get on.
Picasa is also particularly useful for private albums which is something I had to do in the last few months to prevent photos being used. The Private function will allow you to send an email to those you wish to see an album or picture within the confines of your privacy. Do remember tho, people do copy your photos - so don't put up anything that can be used by other people without clicking the "private" feature and you'll see a padlock on the album. Obviously, this can be edited at a later stage if you wish to.