After you place your spray in the ...\logos\custom folder, go into the in-game multiplayer options and select "Import Spray." Select the spray you just dropped in there, and it will then appear in your custom spray list. If you ever add more than one custom spray, choose between them by selecting "Custom Spray" again. A folder will appear with a list of all the custom sprays you have added.
I'd like to shake the hand of whoever marked this topic for "other: beating a dead horse" ~ Cheradenine