1 - Yes, you can have images. NO porn, nudity, advertising for profit websites (like retail stores that don't support Bladeforums directly), excessive gore, etc is not allowed. As far as size, we don't allow giant billboards, they look like crap and are unsightly. Just be reasonable, there is no set in stone size limit, but if the images are too big, we will let you know.
2 - I'm not 100% certain, but it can possibly be both. The information on size limits are located in the "Paid Subscription" link provided above.
3 - The number of PMs will vary. You can archive or delete them and start fresh with however many your membership allows. You can also keep a record of what you send, so that counts as 2 PMs.
4 - You need a Basic to trade, Gold/Platinum to sell whatever you've bought, Dealer to sell for profit repeatedly, and knifemaker/craftsmen to sell what you've made. Basic and registered users are not allowed to sell anything, but they can buy whatever they want or can afford. Details in the Exchange rules, required reading for any activity there.
5 - I recommend using photobucket or imgur for photos. The forum software may be hit or miss at times when maintenance or patching is underway. Spark (site owner and only administrator) does his best with some very tricky issues that can pop up from time to time, but for the most part, things run smoothly, except for when they don't/ We are here to help, so if you have any questions, please feel free to let us know and we will do our best to make your experience here as positive, safe from scammers, and pleasant as possible.