From a cost perspective, your best bet is something used off eBay. If you want something new, it will likely be way overkill. Hauppauge has always had solid capture devices, and the HVR 1955 appears to have Linux support if you get the right one, as do many of their products. This one happens to support HD as well (I doubt you'll find one with Linux support without it), but also has composite capture. NTSC and composite capture are quickly disappearing, as no modern device use those outputs. I've been doing capture for about 20 years now and I don't recall ever seeing a device with just screenshot capabilities, probably because the electronics for full video capture has been so cheap that it might as well be included in any supportable product (it's cheaper than packaging and shipping, so there's no point in not including video capture too).
EDIT: I missed the PAL requirement, for that you'd instead need the HDPVR. Linux support may be limited to MythTV (that's all they reference on the site), but that should be enough to get video and you can grab stills from that. I have an HDPVR--it was pretty popular as it was the first reasonably priced HD Y-Pr-Pb capture device for PCs. It appears to be the cheapest external capture device they sell with PAL support.
There's also this one on Amazon, which reviewers claim works with Linux. It's much cheaper, but I can't find it on their main website, so it must be a European-only product or something.
You'd think I worked for Hauppauge, but I'm just a happy customer. I had 12 TV tuners at one point and 8 of the were Hauppage. They always worked really well for a reasonable price. The other brand I had and liked was SiliconDust, which makes network tuners, but I don't think they have anything with composite or PAL capture.