How afects to my server weight of Images tagged IMG
 

In my new forum People use to upload images on free images servers like image hack, etc. Some one told me that doing this way dont take bandwidth of my server, But the pictures use to be more than 400k, how it affects on my site?. I know that pages loads slow, but it reduces my traffic month quota?

Is it bad for gogle to spider that page?
I've seen a plugin for resize images in IMG tag but not the weight.

On other hand, If i install on my site, my own image server like Vb image hosting, i will be able to force people to upload images on it limiting the max weight, but i think that in terms of traffic and bandwidth it could be worst.
I dont have things clear. I apreciate if somebody can help me about that.