One way would be Krita’s native Reference Image Tool, it looks like a push-pin and is (usually) found on the left in the so-called Toolbox of Krita.
The other option would be to install this plugin from our user @freyalupen:
Plugin - Reference Tabs Docker
You install plugins in Krita by clicking on “Tools” >> “Scripts” >> “Import Plugin from File” and in the dialog that opens, select the file you just downloaded, confirm the selection, then confirm Krita’s activation request and the message that the plugin is installed, and restart Krita.
If you missed letting Krita activate the plugin during the steps above:
Then you have to search in Krita under ‘‘Settings’’ >> ‘‘Configure Krita’’ >> ‘‘Python Plugin Manager’’ for the plugin you just installed and activate it by checking the box in front of it. Then confirm with “OK” and restart Krita a second time to be able to use the plugin.
Michelist
Edit/Add: There are other options based on external software, but I personally don’t like them.