Yes we can, but we "don't modify a post such that it changes the meaning of the post."
Usually a mark is left after editing the post. This can be red or in a small font (Edit: reason..), but it's not mandatory. Moderators are not restricted in this regard.
In this particular case, I reduced the picture in the quote to make the post more readable. Quoting large pictures is not a rule violation, rather it is a matter of aesthetics, as the post looks bulky. I very rarely resort to editing posts, it's mostly about merging consecutive posts from the same user, overquoting, removal of the referral links, etc. In this case, the semantic content of the post itself is not affected.
Thank you both for answering and clarifying my question, I have actually heard about merging posts but wasn’t aware it would show on the post as ‘last edited by…’.
Seeing Xal0lex response that it was an image he edited made me learned something new I never knew since my stay in the forum.