Use this nifty little generator to create an image link you can download or use for placeholders! Or, DIY using this link structure:
https://bakkb.one/[width]/[height]/?f=[filetype]
Enter the width and height in pixels, and the filetype as any of jpg,gif,bmp,webp,png. You can also leave the filetype off to default to png, eg. https://bakkb.one/300/300/ will produce a 300x300 .png file.
"*" indicates required fields