I think the guide would be the best place, as it's an easy and open spot for people to contribute. A similar article I put up in the guide was about how photos are featured. These articles are titled with an "About:" prefix to separate them from general guide content. I'd suggest titling an article about the upload process About:Uploading_Photos to be consistent.
We probably should then also create a section where people can get a quick oversight of all the articles that are about how to do things on Travellerspoint.
Once an article like that is up, I'd intent to link to it roughly like you suggested (directly from the upload screen), as well as from the FAQ and perhaps some other areas as well.
Regarding inserting photos in the guide; the same code is used to insert images as in the blogs. There is an alternative available to insert photos with code like this , where the number is the unique Travellerspoint photoID. That's kind of the default one to use in the guide, but not particularly useful for an article like this - the formatting is not really suited. You won't find the "img" button in the guide either (to avoid confusion), so the code has to be edited entirely manually to achieve that look. Bummer, I know. On the plus side, just copying and pasting your blog entry should work without hitches really, because all that code is supported. Copying and pasting photos in certainly would be nice, but really quite a LOT more complicated to achieve from our end! I'm open to ideas on making the inserting of photos simpler though - it's an area that confuses a lot of people.