Eugene Medvedev

PBP files are not a requirement, they're a convenience. Let's do a proper lecture here, because tutorials are bad for your brain, they teach people to do things without understanding why are they doing them...

RetroArch will read CDs in the normal ISO, CUE/BIN (and even BIN with a lost CUE as long as the CUE was simple enough) formats, IMG (which is actually the same as BIN with a lost CUE) as well as compressed CHD and PBP. (I'm actually not sure if CSO is specific to PSP or not, but nobody seems to use it for PS1, so let's skip it.)

It is generally a good idea to use a compressed disk format, because SD cards are slow, and the less data you have to read off a card to run the game, the faster it will be, even accounting for the time spent decompressing it into memory -- especially considering that RG35XX only has 256Mb worth of RAM, so a full CD image would never fit in there.

CHD is the more generic disk compression format, which is only able to contain one disk per file, but is applicable to any CD format (including SegaCD and PC Engine).

PBP is a Playstation 1 specific variation, originally designed to play PS1 games on a jailbroken PSP. It's a Playstation-specific format, that has the distinction of being able to contain more than one disk in the same file.

M3U is a playlist file. In the simplest variation, it's a plaintext file listing other kinds of ROM file, one per line, allowing you to tell RetroArch that these files constitute a disk set, which makes it easier to swap disks when you need to, and give GarlicOS launcher (which turns every file it finds into a launcher menu entry) a single line to show while hiding all the individual disks from view in a subdirectory. (In case you're up to making an M3U and you use BIN/CUE images, you want to have the CUE files in the M3U, but not the BIN)

You don't have to use any of them. In the long enough run, you will probably find PBP more convenient for PSX, because this way at least you can't lose or forget a single disk out of a disk set.

But just dumping the files you get into Roms/PS will work for ISO , BIN/CUE and IMG, though it might not look as neat or work as fast.

I've no idea how ScummVM core handles screenshots, if it even does. It's very possible it doesn't, though. In cases like these, you can use Screenshot Daemon, and then manually rename the screenshot it makes to match what RetroArch screenshot name would be:

<rom filename without extension, in this case the name of the .scummvm file>-0-0.png

Then Nimshot will pick it up. (It ignores the date and time, so you can just put zeroes there.)

The mask I packed with it covers the left side of the image entirely, so maybe. Try flipping it horizontally?