Alright well right now theres no actual rules, but there is a such thing as common courtesy.
Posting a picture of your 40 inch desktop background is NOT acceptable, lol. RESIZE IT. Most image uploading sites give you the option to resize your image with the click of a button.
Right now if it fits inside a standard message its ok with me. But I dont know the rules of this forum, and how many pictures it will take to exceed bandwidth, but lets not push our luck, keep it reasonable.
A great option, if at all possibly....and it would be great if everyone did this, is to use thumbnails. IF you dont know what it is, it is a very small scalled down version of your image that you can click and it links you to an external full sized version of it.
And again, no porn.