Thumbnails size making dataset navigation slower.

[SergioPesquera]

Hi, first of all, thank you so much for the work, this is a really powerful tool and I think that it has potential to be even better! :)

I think that you should change the way that it compresses .jpg's while making the thumbnails, because sometimes it makes really big ones (I've seen in the code that compression quality is fixed to a "50" value #6 ) for example I've suffered 1MB thumbnails from a 600 images dataset (really small) but it makes navigation on the dataset a pain in the ass...

I see 2 ways of "fixing" this:

1. One of it beeing to fix the size of the compressed file, and not the ratio of compression (since there is no gain in having detailed thumbnails IMHO).
2. The other one beeing a parameter that the users (admin) could change if desired.

Thank you so much in advance, and Great work!

[jsbroks]

Probably be best to set it as an environment variable.

Easy first PR

[felixdollack]

Regarding point 1 mentioned by @SergioPesquera, there was this question on stack overflow that is related and seems to have working code: https://stackoverflow.com/questions/52259476/how-to-reduce-a-jpeg-size-to-a-desired-size/52281257#52281257