All of my photos are saved to Google Photos because I have an Android phone. However, I do not link any photos to a website.
I open Google Photos to the picture I want to post. Take a screen capture. The file size is much smaller. You could use snippet on Windows. On Mac, I use ctrl + shift + 4. My keys are modified so it might be slightly different if using an actual Mac keyboard.
Then click the paperclip option in the menu where you are replying to the thread. A window opens to "choose the file". It's saved on my desktop. So a few clicks and its uploaded.
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