Consider increasing default image size

[sykul]

The tool is great but the images are a little small for me. I guess defaulting to small images is to save on disk space but it would be great to able to select image size.
I'm not so familiar with Rust so I couldn't figure out from the code how image size is handled currently, but perhaps allowing the user to scale the input resolution by a factor of the original would be the simplest way.

[emk]

Substudy does not, in general, provide any command-line options to customize how it works or what output it produces. This is deliberate—if there's a problem, I'd rather try to fix the problem for everyone, rather than having 20 options so that everyone can try to fix the output themselves.

Given the increases in screen resolution and pixel density since substudy was written, it might be worth increasing image sizes by 50% for everyone. Would that be large enough for your use case?

I'm reluctant to go too big, because (1) I don't want to indirectly abuse the AnkiWeb server by having people upload tons of large images, and (2) the images are intended as a visual mnemonic, not as something where the user actually needs to see the details. But I suspect that the current image size is at least a bit too small for modern systems. Thank you for the suggestion!