Based on the extra bandwidth needed I would slightly tend to say no to images. But if they are allowed, then only by uploading them to the server. Because nothing is so irritating as reading old threads with images where links are broken or worse, the image content on an external server has changed.
You could address the speed/bandwidth issue to some dgree by simply linking to them. Another option would be to make it a user preference in the control panel with the default being a link. Most forum software uses a thumbnail anyway.
As the saying goes an image is worth a thousand words, often when I'm looking at video questions I could explain something with an image that may otherwise never be clear even after a lengthy explanation.
Every time i write an instruction for my staff i use 2-3 images. every image saves me 10 minutes of writing. i vote for images in the posts.
@russiandesigners... welcome to webmasterworld by the way!... if you need pics to tell your staff what to do, and can't get it done is 40 words or less, and avoid 20 to 30 minutes of your time, might suggest a different biz plan. No disrespect, just a heads up fact of life.
Above is 52 words (give or take) done in 1 minute.
Despite the current policy I can see times when an image might add to the discussion. I don't think I would be a fan of in-line images being allowed. I might be happier with a link to a WebmasterWorld hosted image that I know would remain in perpetuity.
no to images - unless you are going to spend a lot of time moderating them, not for inappropriateness but for stolen images ... i feel that it would reflect badly on a professional forum.
i also feel the image issue is connected to the outgoing links issue, it is also in the spirit of the web that you should link to content not copy it, so if links are allowed then the page an image is on should be linked to, rather than the image be uploaded to the WebmasterWorld server.