Well, as a moderator, one can certainly resize the image by editing the post and following the same steps, outlined at http://www.flyertalk.com/forum/11973208-post3.html on behalf of the member.

In terms of the maximum image size, 640x480 is a safe bet for most users. The link to the original image should also be added, which would preserve the integrity of the post and also allow those readers, who are interested in accessing the original full-size image to do so, simply by clicking the resized image in the [edited] post.

Lastly, members can be provided with tips as to how to resize their images before posting them -- perhaps by way of a help link or a notice, which would appear when one tries to insert an image...