When you edit a page, you have an option to add a photo. It's in the controls. If you are using the visual editor, it appears at the top and says "image". The source editor has it on the right-hand side. I recommend going through the wikia Help Pages which I already linked you on your wall when you started here ;)
You don't? Categories are set by the page being put into that category. So you would need to go to the main page of that category to change the image. In the case of Objects, that's not going to happen. Honestly, since we have an Objects page already, we don't really need an Objects category.
Along with the new Winter wonderland Biome, there also appears to be some new minifigure themed characters in the game as well. You probably found out yourself but I thought I'd let ya know! I found the new biome while playing and found the new characters on YouTube videos.
Yeah we're planning to make community events like battle of figures where each wiki can choose a Minifigures and vote on the best at Lego community central. Also you can get more contributors from other wikis.
Including your wiki we now have 5 enlisted wikis :D