PixelHunt: You can use the left and right arrow keys on your keyboard to move through the images one by one.
.wojtek: OK, this somewhat helps but still requirems me to switch from touchpad to arrows.
Thanks though :-)
I only use the gestures in Safari when wanting to gradually shift to the previous or next webpage in history, so as to temporarily see a little bit of a webpage without risking a reload of either webpage. Doesn't work in Firefox, gestures only switch to the previous/next webpage without the ability to gradually reveal them.
In general, I find the touchpad does pretty much the same thing as the arrow keys combined with the nearby modifier keys.
There's an Option key next to the arrow keys. Holding the Option key while typing the down or up arrow scrolls the page one window-full at a time. Also, spacebar and Shift-spacebar do the same thing, respectively. I used to reach for the spacebar with my right thumb if I didn't want to move my hand far from the arrow keys, which also allowed for my third and fourth finger to hold the Shift key, if needed.
Holding the Command key (with thumb) while typing either up or down arrow scrolls all the way to the top or bottom of the webpage.
And if it's Safari, then the scrolling is animated, but if it's Firefox then it's just instant, IIRC.
Just be careful with left and right arrow keys. In Safari, holding the Command key while typing left or right arrow will traverse browsing history (backward and forward). Command-Shift and left or right arrow keys switch between tabs in a window. This is much quicker than maneuvering the pointer to switch tabs, and then traversing history.
Admittedly, I have Sticky Keys turned on (System Preferences > Accessibility > Keyboard (near bottom of list)), so I sometimes just type a modifier key to temporarily lock it instead of having to hold a modifier key at all.
All of that seems to work well with left/right arrow keys switching screenshots on a GOG game page because left/right arrow keys (without modifier keys) don't seem to have any effect in scrolling the webpage or otherwise.
However, clicking on one the screenshots on a game page reveals a larger version and the keyboard shortcuts. One the shortcuts is the down arrow key, which opens the screenshot in a new window (or tabbed window). That only happens when the large screenshot view is open, otherwise the down arrow key just scrolls the page.