can't find pictures

• Note: this question being in "Desktops"; I assume you are referring to downloaded pictures or files that were transferred in a specific folder
• In Finder, often people keep in their favorites list (left of window), shortcuts such as "Downloads" and "Recents" that can help you find back those recently downloaded or created files
• If you know those files names, you can also use the magnifying glass (top right of the screen) to search your hard disks
  • Note: if by "browsing pictures", you are referring to "pictures you created with your camera" and then imported them, you can then open the Photos app (in magnifying glass; write Photos and click it, or press enter). If pictures you are browsing are in Photos and need to be modified before being shared, or if you own several very similar ones, you can click on it and drag it in the desktop or specific folder. This way Photos will contain all the raw pictures and the modified picture you posted online with be in a specific folder you decide. Ex: "Documents" folder
  • Note2: Often on MacOS, several web browsers are used. If you downloaded those pictures from the web, double-check all your browsers "Save folders" for these files which can sometimes differ from browser to browser
    • Trick: Download to /Desktop often makes new files or new downloads easy to find and since your place them in a proper folder afterwards, less difficulty find files on your computer

• If previously uploaded in Photos, you can browse: Macintosh / Users/ [*your name] /Pictures /
  • And find the file "Photos Library.photoslibrary" which represents your complete imported photo library
  • To open it, right-click and select Show Package Contents.
• To browse your content, the DateModified column is usually helpfull to find those original files again

I hope this helped.

Cheers!