
When uploading pictures straight from your computer I can't find any way to make the picture bigger than a thumbnail. But There are a lot of free image hosting websites out there. Once you have your image uploaded to one of these sites, Right click it, press copy image location which will copy a link to your clipboard for example "https://imagelocation.com"
You then need to lay it out like this HTML Code:
[IMG]https://imagelocation.com[/IMG]
and It should post in your message at whatever size the actual image was. That's about as simply as I can explain it. This works with facebook pictures as well.