fix(developer): support gif preview images in Package Editor

Fixes #6974.

GIF images were not supported in the Details tab, Image file, because we
didn't link in GIF file support. After linking it in, we also needed to
change over the picture rendering to have an intermediate render step,
as the existing model only worked with .bmp images.

At the same time, added warnings for wrongly-sized images to the image
size information.