Improve image preview and sizing

[hesterbruikman]

Problem

When uploading an image, the user cannot anticipate what it will look like on the project page. Images appear out of proportion.

Current behavior

There is no preview of the uploaded image or guidance on the required image proportions for proper scaling.

Expected behavior

There is a preview of the uploaded image as well as guidance on the ideal image size.
Optional: there is a way to adjust scaling and cropping of the image inside the dapp.