For Linux users who also use Google Photos already, you may not realize this but the Google Photos web app accepts paste from system clipboard via Ctrl+V in the browser. Thus, my workflow if I want to save a screenshot for later is to call up flameshot in rectangular selection mode (I bind it to the PrtScn key), select my screenshot area, Ctrl+C to copy it to clipboard, navigate to GPhotos web app via address bar / bookmark bar shortcut, and Ctrl+V to upload there.
The nice thing about this is that GPhotos recognizes it as a screenshot (so I can find it on my phone later, too, for example). And, GPhotos also automatically indexes any text within the screenshot, so free text search can often find it, too.
If I need the screenshot as a file for some other purpose, I'll navigate to it in GPhotos and use Shift+D to download it.
I can also use GPhotos to privately share the screenshot with someone via their email address, or get a tokenized link for it.
Just sharing this tip because I notice a lot of people hunt around for cloud storage for desktop screenshots. But Google Photos works pretty well for this purpose already, if you use the paste-to-upload trick!