So I'm going to drop a bunch of information on you, hopefully you can find some of it helpful. Firstly, to answer your question directly, PlayStation games play at different internal resolutions. For example, Mega Man 8 displays at 320x240, while Legend of Mana displays at 320x224.
Some games even change internal resolutions on the fly. While Castlevania: Symphony of the Night plays at 256x208(?), the menu selection screen is displayed at 366x240.
There's also an issue with hardware rendered dithering on PlayStation titles. While this doesn't effect all games (and you can even patch it out of some titles), chances are you'll run into it eventually and whatever you screenshot rip won't be accurate. Another issue would be games that simply don't render their graphics at native resolution. You can see both these issues with Strider 2, which despite being played at the internal resolution, still has stretched graphical hiccups and the previously mentioned dithering.
While I wouldn't suggest screenshot ripping a PlayStation title, if it's what you feel most comfortable with then I wouldn't hold it against you. Some emulators such as BizHawk will display games automatically at their internal resolution, so you may find better results there. I prefer to use no$psx for PlayStation ripping as it has built-in tools to view the Vram.
EDIT: Ah, this image isn't displaying properly here because it's so big, right click and open the image from it's source for the actual size.
The problem is that because the viewing area (bottom right) is so small and scales with the window, you have to increase the window to this massive size (1602x1033) just to display it at 1:1. On some monitors like my current laptop, increasing a window to this size is impossible, but I was able to do it on my old PC setup so your mileage may vary. If you get it working though, the results can be pretty exciting.
Anyway, like I said, hopefully you find some of this helpful. The PlayStation is by no means an easy console to deal with given all the issues I've mentioned, so best of luck with whatever you decide to work on!