A general question about posting stuff

Hi all,

I am going soon to be in a position to post something in connection with another author's works.
The content I wish to post is visual, images if you like, and is a conjunct to the work, with, of course, the author's gracious permission.
I have developed it using Inkscape and can dictate the final format.

The most obvious ones are .png or .pdf

What are the size limits imposed by this site?
How do I connect this photo / these photos to another written thread?
Are there any other constraints about which I should be aware?

Thanks for the help

Di