How can I post a picture of a painting to find out if anyone recognizes the location?
Is the image located on a website that you can post a link to?
You can also try yourself with google image search.
https://support.google.com/websearch/answer/1325808?hl=en&co=GENIE.Platform%3DDesktop
Several years ago, someone posted a picture of his grand dad on a bridge in Paris during WWII. He was trying to find the exact location. He put the picture in dropbox was able to share the link here. Got a match the next day.
Put it on Imgur, copy the link for BBC Code, and then drop it in the thread. Like this:
If you have a Google account, you can upload it to Google photos (unless your photos already automatically go there), then click on Share Photo and it will create a link that you can insert into a post here.
Several good ideas posted already.
Very easy is to use ImgBB because no account is needed, and the uploaded picture is automatically deleted after a few days.
After successful upload the tool will show a link which can be copied by a click and published here (press STRG+V).
My example picture of a Danish flag: https://ibb.co/PDZ0T3X
Curious about your paintngs' motiff.