MediaWiki supports some basic formatting and captioning for images that are uploaded to the platform. This is not the case for externally hosted images.
My work-around is to use this simple MediaWiki table template:
{| border="1"
| ImageURL
|-
|style="text-align:center;"|'''Figure X''': Caption Text.
|}
I simply add the URL to my image and amend the caption text as required.
I am leaving this here so that I have a reference to copy and paste from.