HyperPin was the original front-end for cabs. It's the one that defined standards for media that are still used by all the various front-ends that are active today (PinballX, PinballY, etc.). It only supported swf for instruction cards. Why is that? Because Flash can display vector-based graphics that are scalable to any size screen. After all, you are projecting the contents of around a 5" x 3" card to a 2K or even 4K backglass screen that is 24", 27", 32".
If you are dumping a jpg into a swf container and that source is really good quality then it's probably acceptable. But I suspect that if you could not find a media pack for a table that you have installed then whatever you've found is not so good. Jpg has not been a standard for instruction cards that I'm aware of.
I know that you are just trying to tick off an item on your media audit and move along to the next item. But look in your heart and tell me which would you rather have blown up and displayed on your screen:
This - an original resource photoscan that is legible:
Or something like this - an Inkochnito repro:
This - a photo of a card:
Or this - a repro I put together which is available at the universe:
It's your machine. You do what you want. But at the end of the day, if you are going so far as to set everything up in a front end, you know that you are not going to be entirely satisfied with resources that don't look very professional. I've redrawn to Flash many, many cards from Inkochnito's site. They are available on the PinballX FTP. But I don't have as extensive a collection as you. Chances are Inkochnito has the reproduction. Clipping from one of his PDF and converting to vector is a pretty quick and easy process. It was how it was done for the older Hyperpin media packs from around 2012 or 2013. There is, I think still, a greymarket version of Adobe Suite that Adobe had made available. It's ancient but it is free and it does the job. Much better results than dumping in a photo or a tiff from a scan.
If Inkochnito does not yet have the reproduction. Well, you'd have to reproduce it yourself but that would be a separate tutorial.
It's true Flash is going to go extinct. I've looked around at what might be a good replacement standard but I'm not sure what that would be. I looked at svg but file sizes blow up and because a lot of the modern Stern cards use a mix of vector and bitmaps, it wouldn't be appropriate anyway. Png would be my fallback as well. I've got an AutoHotkey script that runs a quick slideshow and takes ultra hi-res screenshots. It has the advantage that it will display max res while retaining the correct aspect ratio and letterbox the areas outside the image. Of course, the output would not be transferrable to another machine running a different aspect ratio. I have a 16:9 but the image would be distorted if you tried to display on 5:4 or 16:10 backglass displays. I think TerryRed has actually published some conversion programs to support Popper. And there maybe something useful for swf cards in his collection of programs.