....Linux you're entirely at the mercy of the selected distro and the window manager (desktop environment etc.) that you're currently running.
While Mark is definitely right, there is no certain answer. But a pretty good guess on a desktop system is ~/Pictures - don't assume its there, always check, but its there in all my VBVMs including Debian (and Ubuntu, Mint, MXLinux), Fedora, Manjaro. The major Desktops seem to agree ...
Maybe someone can confirm Arch and SUSE.
~/ refers to user home directory, get that from the $HOME env var.
Not certain, but a good place to start. Mind you, its Linux we are talking about, just because the directory exits, the user may like to put their pictures somewhere else.
Davo