Translation:If the photo were more beautiful, I would download it.
I don't think so. Anybody who understands how computers work knows this. How can you display information that you don't have? Of course you have to download it first, technically.
The colloquial sense of downloading is saving it (not in RAM, not in a temporary folder where your browser might delete it to gain space, but in a place where you will find it again and it won't disappear after a while).
How would one use conditionals in a past form?
If I would have downloaded it...
(Si lo hubiese descargado...)
Mi serĉis kaj mi trovis tion: http://esperanto.davidgsimpson.com/eo-verbforms.html
What do you mean? Are you saying that "Se la foto pli belus, mi elŝutus ĝin." means both “ If the photo were more beautiful, I would download it.“ and “If the photo would have been more beautiful, I would have downloaded it.“?
(English doesn't help in this case)
[Si la foto fuese más bonita, la descargaría.] ≠ [Si la foto hubiese sido más bonita, la hubiese descargado.]