Yes, you take in-game screenshots using the C key. You can use the Tab key to go into movie making mode but is also used to take screenshots as well. The game/camera controls are:
https://strategywiki.org/wiki/The_Sims_3/Controls
To view the screenshots go to Start Menu > Documents > Electronic Arts > The Sims 3 > Screenshots
This might be different with a digital version, Macintosh computer or if using a SSD (Solid State Drive).
Then you need to find a 3rd party image hosting site to upload the screenshots there before you can upload to the forum.
https://en.wikipedia.org/wiki/List_of_image-sharing_websites
Once you have your screenshots uploaded to the image hosting site, then there will be an option to share photos. You copy the direct link and paste it on this forum under the image tab as you are writing a comment. As an example, it should look something like this except with before and after the link:
https://i.postimg.cc/FsZvdxrh/Screenshot-90.jpg