To add on to image embedding images as Alessia Starfurr has so wonderfully explained above - when embedding, the image will remain up indefinitely, unless the image itself is removed from the original host site, or the URL of the image changes. (I.E if you post an image to Tumblr, then copy the image link and embed it to a forum post, it will stay on the forums unless the Tumblr post containing the image is deleted.)
Your embed should look like this:
You will know when you've done it right, when you select 'preview', and it comes up at the top of the page with your image! make sure to scroll all of the way up if you can't seem to find the preview.
It is advisable to self-host the image if you want it to remain, or to use a site such as DeviantArt's Sta.sh to store the image.
I cannot speak for the hosts of this forum; but as an owner of a different forum myself, embedding is often preferred as it saves on server storage - hence why there is an upload limit on image size and number of images
- you may have to learn the ancient art of JPEG compression if you wish to stay solely to image attatchments VIA upload
Happy foruming!