If I know what I am doing, I have increased the maximum size of an attachment to 1 megabyte. So now you should be able to upload a very large image. That should help when you are selling stuff or asking for help with a problem. (That is quite large... your full size computer monitor is only 2 megbyte image !)
There is a limit on # of bytes of attachments per posting. So if you want to load a few 'large' images, you need to make multiple postings.