picture resampling [feature request]

[chrispable]

When sharing photos, I frequently get a message that my image is too large, so I have to load another program, resample the image and then start over posting again.

It might be nice to query the maximum size then compare the the users supplied content and ask to resample when applicable

[ford--prefect]

An alternative would be to allow incoming share requests (as proposed in #11) from other apps (e.g. send-reduced), which do the downsampling.