You have to have a link to an image, insert it with either the Insert Image button on WYSIWYG:
or if you don't have that toggled on, you'll need to use HTML: <img src="URL">
For either of them, you have to insert the direct link to the image, not the webpage the link is on. It must end in .jpg, .png, or .gif. Pasting "https://toyhou.se/440245.-perpsona#9229709" or "https://imgur.com/lIejaVR" will not work, but "https://file.toyhou.se/images/9229709_PoyQk8XXh98AkoO.png?1546549195" (the ? here refers to the watermark and is ok, as toyhouse knows what this means) or "https://i.imgur.com/lIejaVR.png" will be fine, and will show up properly. You can get this direct link by right clicking on any image, or if on mobile using a long-press, and selecting "copy image address" (on chrome) or some similar phrase (on others).
Use a permanent host like imgur, discord, or imgbb, and not stash, furaffinity, froala, any apps, or "local" locations. If your image appears for a few hours or days and disappears afterward, the host is temporary to prevent hotlinking.