We have two kinds of ads on PPRuNe - those directly aimed at the aviation community (VA, Ryanair, multiflight, ...) - I'll call thse "type 1" ads - and more general ads such as the BT and Dell ads - I'll call these "type 2" ads. Some forums have a type 1 ad at the top of each page, some have a type 2. Every forum also has a type 2 ad at the bottom.
All of the type 2 ads (e.g. BT, Dell computers etc) have the .gif stored remotely. We don't know where, and we don't care.
Some of type 1 ads have the .gif stored on our server, some are stored elsewhere. We prefer it if these .gifs are stored on an advertisers own website as then if they want to update it they can do so without requiring us to do anything. Type 1 ads all use pretty much the same HTML.
---PPRuNe Dispatcher