The problem can be in missing GraphicsMagick library. You need to install GraphicsMagick (default) or ImageMagick, both libraries are multiplatform.
I had same problem on macOS with GraphicsMagick. Try to switch GraphicsMagick to ImageMagick in app /config
file:
default-image-converter : im
Then don't forgot to restart app.
Eshop has a small protection for testing credentials. If some user performs more than 5 attempts to login then the system blocks IP for 15 minutes. So maybe is your IP blocked.
Uploading files is limited according to rules below and you can extend this size in: /controllers/admin.js
file.