Here is a neat tip on how to get the most out of the wallpaper gallery by using the dA search feature to single out specific resolutions.
Firstly, you need to select the Customization > Wallpaper gallery in the Browse feature.
Next, you can select a specific section of the gallery if you want, for instance Customization > Wallpaper > Fantasy
In my example I have also selected the "Popular" and "All Time" sections, but you can browse by day, week, month, popular, newest... the possibilities are endless.
Now all you need to do is to enter the resolution you wish to filter by in the Search box at the right of the page like this:
This is the filter search for the page (not the main search at the top of the page which will search all of dA).
Simply place the resolution with no spaces, as number x number ie 800x600 or 1024x768 etc, and then click the 'search' button and the page will show the filtering like this:
Then you just have to find a wallpaper you like. Unfortunately this is not something I can help with and from this point on I'm afraid you are on your own...
(many thanks to AreteEirene for pointing this tip out to me)