As great as I think Flickr is, I still wish they would fix or add two things:

1. Black thumbnails: Whenever I upload an image bigger than maybe 4000 x 4000 pixels Flickr is unable to generate a thumbnail, and the picture appears as a black square except when viewed in its original size (see this example.) It's in the Flickr FAQ, so it's a well-known issue.

I realize that Flickr is primarily a photo site and that most users will be uploading images in the 5-10 megapixel range, but I really wish they would fix this.

2. Toggle private photos in photostream: I wish there was an option to not show my non-public photos when I browse my own photostream. I regularly upload images that I want to have easy access to but which are not intended for public viewing. Often these are batches of up to 30-50 images. After uploading a private batch I have to click through pages of private images to get to the public ones.

I know I can just sign out to see my photostream as other users would see it. But that seems kind of, well, stupid. A simple option to filter by privacy setting would be a great help to any Flickr power user.

Always save some sugar for last: Flickr recently introduced a nice new feature called Collections, which tackles the issue of having a lot of sets. Collections allow users to organize their photo sets so that related sets can be grouped together. Have a look at my Collections for an example.

Collections get a mosaic of images as icons, which can be randomly generated or painstakingly hand-selected. There is also a new option to choose the layout for your photostream to display collections rather than sets in the right-hand column. This is the first time users get any say in the layout of their Flickr "home pages", it would be interesting to see if more customization features get added in the future.

I only wish there was an option to simply have single images as collection icons. The new photo mosaics give a decent impression of the contents of a collection, but they can look a bit messy and not as attractive as a single good image.