So... It looks like the maximum image size you can upload is around 1MB ( on3 help says 2MB but I can't get anything but 1MB)
And if you want to link to larger than 1MB remotely hosted media (hitting the : button next to the image button, then the image icon again) you can add media with links from these sites only:
Approved sites: ABC News, Acast, Amazon Product, Anchor, Apple Music, Apple Podcasts, Audioboom, Audiomack, Audius, Bandcamp, BBC News, BitChute, Bleacher Report videos, Brightcove, Castos, CBS News Video, Clyp, CNBC, CNN, CNNMoney, CodePen, Comedy Central, Coub, Dailymotion, Democracy Now!, dumpert, 8tracks, ESPN, Facebook, Falstad Circuit Simulator, Flickr, Fox News, Funny or Die, Gamespot, Getty Images, Gfycat, Gifs.com, GIPHY, GitHub Gist, Global News, GoFundMe, Google Drive, Google+, Google Sheets, Hudl, Hulu, IGN, IMDb trailers, Imgur, Indiegogo, Internet Archive, İzlesene, JSFiddle, JW Platform, Kaltura, Kontinental Hockey League (КХЛ), Kickstarter, Libsyn, Liveleak, Livestream, Mail.Ru, Medium, Megaphone, Metacafe, Mixcloud, MLB, MRCTV, MSNBC, National Geographic Channel, National Geographic Video, NBC News, NBC Sports, NHL Videos and Highlights, NPR, The New York Times Video, Odysee, Orfium, Pastebin, Pinterest, Podbean, Prezi, Reddit threads and comments, Rumble, Rutube, Scribd, Sendvid, SlideShare, SoundCloud, Sporcle, Sportsnet, Spotify, Spreaker, Steam store, Stitcher, Straw Poll, Streamable, Streamja, Team Coco, TED Talks, Telegram, The Atlantic Video, The Guardian (obsolete), The Onion, TikTok, TMZ, TradingView, Trailer Addict, MyDailyFreedom, Tumblr, 247Sports, Twitch, Twitter, Ustream, VBOX7, Veoh, Vevo, Video Detective, Vimeo, Vine, VK, Vocaroo, Vox, Washington Post Video, Wavekit, Wistia, WorldStarHipHop, The Wall Street Journal Online, GameClips.io, Gamer DVR, Youku, YouMaker, YouTube