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Some common wallpaper sources are mentioned: image search engines, art galleries, wallpaper sites. These are compared based on several factors. The best way to find them is provided below.

Search engines

In the first place you could search for images on Google. But it won’t get you too far.

Search engines index large collections of images. They give relevant results for the keywords you search for. But, that’s not the real problem here, it’s the rating. PageRank, what Google uses, might work well for web-pages, but when it comes to images things change. A highly ranked page may have images from various categories, most of them not displayed for aesthetics.

Art galleries

There are lots of image directories, community reviewed mainly. The most popular that also have a good ranking in-place are presented below. All the characteristics have been established based on the “top ratings” listing on each site. Here’s a table with the caracteristics of several sites.
After careful consideration the best wallpapers you can find are on on deviantart.com. They don’t always come in the best sizes but at least you get a nice experience from viewing beautiful pictures.

Imperfect ranking

On art-galleries an image’s quality is decided first by its thumbnail, than viewed at a medium size and only after that it’s judged by its full size. A low percentage of the users get to see that. This is why even with many votes the rating is not perfect for the full size of the image. This problem is more visible when an image has fewer reviews. It happens especially on flickr where an all-time ranking is not available.

Wallpaper sites

These don’t generally have many images to choose from compared to the previously mentioned sites. Even if the pictures are ranked as best as they could (and this is not the case) there would still be a lack of very good quality images. See here some examples of anime wallpaper sites.

Searching for a wallpaper has it’s trade-offs. You cannot find keyword relevant, good quality, diverse and perfect sized wallpapers all in one source.

Found a beautiful image? See how to resize a wallpaper online with the online image resizer from Reshade.

Other wallpaper sources:

a list of free wallpaper sites
top 5 free desktop wallpaper sites

A quick definition of stock photography might be helpful here. The wikipedia entry says that it consists of existing photographs that can be licensed for specific uses. Book publishers, specialty publishers, magazines, advertising agencies, filmmakers, web designers, graphic artists, interior decor firms, corporate creative groups, and others use stock photography to fulfill the needs of their creative assignments.

There are many big stock photography sites online. A quick search on google returns popular names like corbis, fotosearch, comstock. The businesses prosper by selling photos at different sizes, as the client specifies. The prices for these sizes vary to suit everyones needs: for small images the price is low, but these can mainly be used only on websites and other mediums that don’t require great resolution. The expensive large sizes are suited for printing, displays, posters etc.

We’ll talk more about this price - size dependence. Here’s a plot depicting this:

The blue line represents a plot of the price of an image found on stock photography sites (average price for the mentioned sites) versus the image size (in megabytes). The red line represents the best linear fit. This gives a trend for estimating larger size prices. The megapixels of the photo can be calculated by dividing the size in megabytes by 3.

A more detailed view of the data used

The resolution of the photos can be derived from their size (in MB) like this:

640 KB - approx. 640 x 480 pixels; 8.9″ x 6.7″ at 72 ppi
2 MB - approx. 1024 x 1280 pixels; 14.2″ x 17.8″ at 72 ppi
14 MB - approx. 1700 x 2550 pixels; 5.7″ x 8.5″ at 300 ppi
32 MB - approx. 2800 x 4200 pixels; 9.3″ x 14″ at 300 ppi
50 MB - approx 3400 x 5100 pixels; 11″ x 17″ at 300 dpi

Using the linear estimator from the figure we can calculate the price for a 150% and 200% times larger image. These factors are considered in the zoom column from the table below. The megabytes have a quadratic dependence on the zoom factor: Let’s take the 200% zoom (from 8 to 16, respectively from 60MB to 240MB). This means that the new size will be 200% the width * 200% the height of the photo. This gives a 4 times larger photo (60*4 = 240 MB).

Here’s a different application of reshading. I used it on a small video and seams to work fine. YouTube uses a standard resizing method. There’s lots of blur resulting from this. Reshading the video creates noticeable improvements in quality. The videos have been upsized and rescaled from 153×142 to 425×350.