The
OP , although apparently asking a very specific question that seems repetitive, is a wave with advanced users or those who like to incorporate "1000 more words" with an image to their publication.
I suspect that he is a user who frequents the sites that "resize" the photos, so common that when they come to a forum like bitcointalk.org it seems impossible. It is not that this is uncommon, e.g. in Steemit, a simple command is required for very basic things.
That said, let's "dismember" your questions:
... Could someone help me what exactly is the size of the images to post in thread?
No. How to know exactly! each post is personal-individual.
Here we enter the topic of suggestions and that is why I take your post, because in the general sense several responded and some answers are helpful.
One of the reasons that I answers, that you cannot specify what the size is, it depends on the objective of the image in the post:
If it is a header it should be a 700x200 or 700x450 photo. Specifically, standard photos that come with a size ranging from 1280x720 are used.
Why not use bigger or smaller ones; small it is obvious that a photo that starts a publication does not look good,if you are targeting a message.
The image does not contribute do not use it, in a forum like this the images should contribute to the content not decorate.
Contribution example:
https://bitcointalksearch.org/topic/m.54802650Decoration example:
https://bitcointalksearch.org/topic/m.54632811In your OP you mention something important "... Suggest me the best
size and resolution for image."
If you try to use a very low resolution image and want to enlarge it it looks bad, they say "pixelated." The example in post # 4 of mk4, is the best example from its original size to a smaller one, nothing happens, but if the resolution is low when enlarged it is distorted.
[img height = 600]https://bitcoin.org/img/icons/opengraph.png[/img]
So keep in mind the resolution of a photo if you want to resize it.
Another important thing is using the
button, that gives you an idea of how it will look, keep in mind, that the size of your monitor may be not average, that is, you either use a very large monitor or work from a table. (I would never try this from my cell phone;) )
Finally, the above can serve as an example of two additional uses that I can "suggest" of use in size and resolution.
Additional:
The use of images is limited to going alone on one line, you cannot include text on the right or text on the left.
I tried to use this command:
[imtext =] would be an explicit ornament in a post [/ imtext]
There is additional code that works over html, but that option is locked.
. The important thing is you copied the html link of the uploaded photo and you're good to go. 
