Attachments are limited to 375kb each, 1500 total (four images per post).
Currently the max width for posted phitos (i.e. linked from Photobucket etc.)
is 1200 pixels, but all,that means is that it'll be downsized for viewing.
If you are using the attachment feature you'll need to adjust the image size rather than the resolution.
Generally a width of 800 to 1000 pixels at 72dpi is more than adequate.