I see a lot of confusion on this thread, especially caused by the discussion in the beginning, and later posts not being crystal clear.
Trust me if I wouldn't have been able to do this myself, I wouldn't be able to teach you how in a simple crystal clear manner.
So, How to do it?
To paste an image inside a post, either use the 'img' tag or use the 'insert image' option in advanced menu. They're basically same. The end result would look something like this - Ex: (img )www.upload.com/image.jpg
(/img), Except instead of round it will be square brackets.
The crystal clear part
is, that you need a full link to the image i.e. the link that will end in the extension of your image, it be .jpeg, .png, .bmp etc. For ex: If you uploaded an image to Imgur, instead of using the url link, use the actual image link, you will see it when you move your cursor onto the image, copy it and use that instead, otherwise you'll get a broken link icon like this -
******That's it. If you wanna learn more, keep reading.******
And just for the sake of testing when I made a post earlier I used .png image, which are always quite large in size as compared to .jpg/.jpeg. And it still worked, so as far as I can tell, there's no size or extension constraints, but I wouldn't push it. You can see it here in action - https://forum.xda-developers.com/poc...ingke-t3974743
Also here is the image to prove that it was png -
Sometimes you may not see the full link even while pointing at the image, which happened to me while making this post, just add the extension of what your original image was at the end of the Imgur url and you're good to go.
As far as the size goes, you can't change that within the advanced menu, there are no flexible options here. So you'd just have to preview the post to see how it looks on your device & adjust the size of your original image you want to upload, accordingly and hope it looks good on other devices as well. As far as on PC, when I 'zoomed in' the browser, the size of the image remained the same, and shrunk when 'zooming out' goes quite low, as expected. I don't think %age resolution applies to such images inside the post.
Also if you want to add a name or comment to your image, you can do that by adding text to the first image tag Ex: (img ="Image name"). You know the drill, square, not round. And when someone rests there cursor on the image, they'll see it.
A little bonus for you -
And YES you can do it all on PC
(what I used). And probably on mobile as well.
Hope this helped you out.