Can I upload an image or a document like a PDF to use on the blog?
Yes. Follow the instructions at Movable Type in order to do this.
When would I need to upload a file?
You might want to put an image in your post or link to a PDF that is not currently on the web.
What size should the image be?
You should prepare all images for the web with software that allows the file size of the image to be reduced.
It is recommended that portrait style images not exceed 230 pixels in width. Landscape style images should not exceed 320 pixels in width.