I have just revised the maximum image and file sizes for image attachments in posts:

Maximum width = 800px
Maximum height = 600px

Larger images will automatically be resized.

Maximum file size for JPG, GIF and PNG is 250KB

Obviously, this doesn't mean that your images *have* to be those sizes - it just means that you can go up to that if you need to.

See this thread if you'd like to know how to attach images to your posts.