Once you upload your photos to Photobucket, there's a menu that you can click (a little cog symbol as you hover over image thumbnails) and in there is an option to "Share Links". Look for the one labels "IMG code", click it to copy it to the clipboard, then simply paste it in a reply to a forum topic.
If you want to see how it should look, go onto someone's project and click "Quote", you'll see the raw text code there (but don't click reply!). Hope that helps.
As for things to look for on the mk4 GTI, I have no idea, I#m a mk5 GTI guy.
