Adding to that, I don't think i've run into the pixel limit
Inline images are limited to 640x640 pixels.
so that bit of info might be out dated.
I host images on Google Photos whichy has an annoying "feature" that linked pictures are only visible to other users if they're logged in to Google. There's a neat workaround:
Go here
https://www.labnol.org/embed/google/photos/ and paste the shareable link from Google photos into the box, click the button, then paste the resultant url inbetween [img] tags on here.
For an embeddable url from Dropbox, upload your image to your Dropbox account, grab the share link, paste it between [img] tags and edit it slightly, changing the final "dl=0" to "raw=1". No, I don't know why, I'm just repeating what I've been told.