Reduce image size for uploads

[chaoran-chen]

As a bonus to #53, the frontend app could automatically reduce the size of the images that should be uploaded.

[michael-kamel]

We should take into consideration the fact that the aspect ratio has to be maintained, and thus the user may not be satisfied with the result of the resizing.

Furthermore, if we resize, we probably don't need to check for actual size of the image. For the extreme cases (which are most probably intentional) the backend will reject the request.