*EDIT* This process is no longer necessary. You can now search for specific resolution by choosing the "More" drop down on the toolbar, then "Diversions", and then "Wallpaper". From there you can refine your search to specific resolutions using the drop down menu, and category by using the category tree on the left of the page.
When v.5 came on-line, the old limit gellery by resolution button went poof, causing great consternation among many users. It's on the list of issues to be dealt with, eventually, but most people don't know that you can use the search to limit galleries by resolution, and you can pick any resolution you like, not just the standard ones that one used to be able to choose from. Plus, you can limit pretty much any gallery you like, not just wallpaper galleries.
Moreover, I am quite sick of explaining in depth how to search by resolution to everyone who either posts in the Help or in the Suggestion Forums (I'm not sick of the posts, just of all the typing ). There's almost always a thread about it, too. So I sat down today and made a tutorial on just how to use the search, for all those who miss the function.