With many themes, they come with a stock wallpaper, that goes best with the design of the theme. Personally, I like to change my wallpaper every few days to keep my homescreen fresh. One of my favorite sites for doing this is imageboard scraper. The imageboard scraper takes the images off many image boards and compiles them into a nice webpage that is search-able and customizable. The unique thing is, it is ever changing as the image boards get new images. They are not put into categorized, but there are plenty to choose from.
There is a nice search tool at the top that enables a keyword search for all the images it has scraped, and also lets the user choose how far back you want to search(time period). The site also has a filter for NSFW(Not Safe for Work) which enables safe browsing. The Raw list lets you set how many you want to display per page. It also lets you determine your own sorting method, so once you familiarize yourself with the site, searching becomes faster and more efficient. One thing you will have to keep in mind while browsing these images, is that there is no next or previous page link, but the box beside the Raw List entry box is a page # entry spot, so you can choose yourself which page you would like to browse.
All of the wallpapers posted are desktop wallpaper quality, so the quality will always be the best it can on a BlackBerry or other devices. I hope everyone enjoys!
Dislaimer: you should clear your browser cache and cookies before opening this site on mobile, and then once it loads, change the Raw List to 10, this will only display 10 images per page and you will not get the Page is too big error in the webkit browser.
A few of my favorites: