Patches are images that are commonly used in game menus and intermission screens. Like textures, patches can be replaced with TGA, PNG or PCX images. The Patches resource class directory is searched using the lump names of the original patches. For example, to replace the Doom menu title, you would place the file m_doom.png to the Patches directory.
The original data lumps are required even if an external resource is found, because the original data includes information about offsets and the on-screen size of the patch. This means the image from the external resource can be of any arbitrary resolution: it will be scaled to match the original patch.
Currently external patch resources are not precached, which may cause slight delays when the patches are first loaded.
You can look around the forums and the Add-ons page. You can also download a rollup of common replacement graphics (HUD, Title screens, intermissions etc) by various people in the jDoom User Interface Pack (jDUI).
Some games (such as Doom 1 and Doom 2) use the same names for specific patches even though they are different in each game (a good example being TITLEPIC).
To get around this simply place external patches in a subfolder with the same name as the game mode you are playing. For example for Doom 1 you may place all your patches in the root Data\jDoom\Patches folder whereas for Doom 2 you can place it's unique patches in Data\jDoom\Patches\doom2.