If you use Photobucket.com to host your images it will resize your photos automatically for you. There standard size is 800 X 600.
And that seems a good size for this forum as well.
Limiting a picture to 800 pixels across still leaves room for the various site and browser elements even on a 1024 size screen.
When people post >1024 sized images, it screws up the horizontal scrolling on most displays <1280 wide.
besides, the file size limits tend to encourage a smaller display size anyway.
So I'd say:
Posts in the body of a thread - max 800x600 (or at least limited to 800 wide)
Anything larger, link to an off-site host.
In general that's the guideline I've tried to stick to in my posts.
The other guideline is whether to link off-site or attach.
If it's core to the message of the thread, and ML or direct audio related, I tend to use attachments, as they will be persistent and consistent across time.
I hate digging through old threads and run across posts with links to images that convey the 1000 missing words and the link is dead.