Is my picture uploaded?
No. The picture is read in this tab and the text is translated by the translator built into your browser. The only request translating causes is the browser fetching a language pack from its own vendor the first time you use a pair, and that request carries nothing of your picture.
Can I paste a screenshot?
Yes. With the page open and nothing else selected, paste with Ctrl+V or Command+V and the picture is taken as if you had chosen it. A screenshot is close to the best case for the reader: flat, sharp and evenly lit.
How good is the translation?
It is your browser's machine translation, and this site has not measured it. The reading has its own doubt, and a block the reader was unsure of is marked doubtful with its score. Read anything that matters against the original, correct what was read, and translate that block again.
Which languages can it read in the picture?
English and the other Latin-alphabet languages, Chinese and Japanese with the reader that loads by default, and Russian, Ukrainian or Belarusian with a second recogniser chosen in From. Whatever was read can be translated into any of the thirty-nine languages your browser's translator lists, where the browser offers that pair.
Can it put the translation back into the picture?
Not yet. Setting it back into the picture is the button under the result, and it is in preview: pressing it counts a vote for building it and charges nothing. Today you can copy the translation or download it as text or Markdown.