1. Is it possible to show number of votes for each photo in Gallery view?
we show a rating in gallery view in showgallery yes
2. It would be nice if rating is shown only after specified number of votes (i.e. only when some image has 3 votes or more).
I take it this sounds like a suggestion. The rating is shown whether there is one or three rates
3. Do you plan to add some features to improve view of photos? I mean, for example, possibility to quick change background to black, white or grey. It's really needed because some photos looks better on black (white) background. Also, the very useful tool is creen cropper (to point author which composition is better for his/her photo).
well there is a limitation of what you can do to photos on the web. Most graphics tools included in graphics programs you do not find in the web since we are limited to the functions in gd2 or imagemagick manipulation tools. We are looking at adding what features we can we do not have but can not say what
4. Admin panel should be improved. I'd like to view more detailed info about current user activity (why don't you make hyperlinks to currently viewing photos/galleries, not just blind "showphoto" or "showgallery").
You might be dealing with the wrong script or area as our online screen clearly tracks where users are
5. Is it possible to ban some users in gallery? Or to restrict their rights?
There is no ban feature in a standalone version. You can restrict users by placing them in a usergroup that you restrict access permissions on.
6. Why private user statuses in vBulletin forum (I mean "invisible") are not inherited in PhotoPost?
I guess the developer of the gallery has not done or checked for that.
7. Sometimes when user adds a photo to a gallery, this photo become visible on the recent pictures page only after a day or two? It IS visible on selective view (i.e. for last 7 days), though.
Not sure what your saying as when a user uploads a photo it should be viewable in the recent pictures immediately unless you have photos set to moderate and you take a day or two to approve it. Non approved photos do not appear anywhere.
8. How can I view ALL photos added by some user? When I click on user's name and then on the Pictures button, I get recent, most popular, and random photos. Where is an option to show ALL added photos? I know about search by name, but is it possible to show all photos without using search?
when you click users gallery under their name you get their gallery
All user galleries are shown under members gallery
9. Is it possible to simplify the way to download photos by users? Adding them to Favorites and then download as ZIP is not obvious way.
Nope not really you can download your favorites or a specific category of photos is how it is setup