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I didn't know this one off the top of my head. If you use steemitimages to host the image, you can add a https://steemitimages.com/{height}x{width} to the start of the url.
To make this sequence of images:

<div>
https://steemitimages.com/0x0/https://steemitimages.com/DQmZBhZLPhqKiHZmLW2tFupUXCtM9dPP4mh85DtLGSwxrrm/171005_Masterpost_Icons-01.png
https://steemitimages.com/100x100/https://steemitimages.com/DQmZBhZLPhqKiHZmLW2tFupUXCtM9dPP4mh85DtLGSwxrrm/171005_Masterpost_Icons-01.png
https://steemitimages.com/50x50/https://steemitimages.com/DQmZBhZLPhqKiHZmLW2tFupUXCtM9dPP4mh85DtLGSwxrrm/171005_Masterpost_Icons-01.png
</div>

Fantastic!!!! Thank you.

the best contribution with which I could come across. Many thanks!!

tsss markdown sorcery!

Whoa! That's a great trick!

This is great - I've always wondered how to do this and now I know! Thank you