If you have the latest update to the PS3, you can do it right from the XMB... you can post pics from the PS3 to flickr and facebook now I think. The way I do it is to stick a USB thumb drive in the ps3 and then choose 'copy' from the menu for the pictures.
I create all my own sigs.
Needless to say, it's not for the faint of heart.
You can take some criticism right? No offence but in my personal opnion these wallpapers are really poor, I wouldn't really call those wallpapers. Most of them are just pure screenshots from the game with basic text and borders added, wallpapers like that, with no "real" editing are just boring. Take a look at the wallpapers JoninRock64 created, that's something inspiring and it should also give you the general idea of "wallpapers".
I really like them. The purity does not detract from the beauty of the screen shots. But then I like JoninRocks' too.
But to compare them is like comparing Andy Warhol to Robert Capa.
i like plaza screenshot that u took