The explanation is given in the "help" section of the JRA website and has been reiterated many times.
I was a slow learner in this department too, so sympathetic!
Here is a somewhat more wordy explanation:
Pasting image into post
I am not sure if there is a file format which can be pasted directly into a post, I never found one by trial and error, but I found pretty quickly that copying images from my own member photo album works, so I keep a folder for this sort of thing and just upload to my album any image I want to use in a post.
When you do your post, it pays to have two instances of JRA running at the same time - one for the post you are composing, and the other so you can hop across to copy from your album, then hop back to paste the image into your post. It will likely be too large. You then need to highlight the image and look for the little corner handles which, for some reason are not easy to find and sometimes well above the position of your image. It is necessary to do this, to drag a corner until you have sized the image down to a size which fits (with a bit to spare) into your post field. (If you don't do that, then when you confirm your post it will flow over the edge of the field and parts of it will be unreadable.)
Linking to a URL
If you want to link to a URL, you highlight the key word then go to the control panel above and select the little link icon.
Paste your URL link into the appropriate field
and the job is done.
Linking to a photograph in your own member album
Here is the trap which catches everyone first time. You can't just hop across to your member album for the URL which leads to your photo. If you do, the URL will include the component /Sys/Profile/Photogallery/ etc
You will think that has worked, because it will work for you. But no-one else will be able to open the link because it is directed just to your private profile.
Instead of going to your "Member photo albums" you must access your image via "My directory profile". The URL will then have the component /Sys/PublicProfile/ etc
Its the same image, but now the URL includes the words "PublicProfile" and you can know that other people will now be able to access your image.
As before, it will save time in the end if you have two instances of JRA website running at the same time, hop from one to the other while making your link.
Later on, if you again forget what to do - just go to the "Help" section of the website to refresh your memory. I must admit I had to make more than one visit.