While I started a decko server with the local decko path, it showed the error of NoMethodError (undefined method 'notable_exception_raised' for # ):.

After cloning the decko code from github, there are several submodules that need to be initialized. You may run the command git submodule update --init --recursive in your local decko root directory. Then redo the seeding bundle exec decko seed.

I have a problem to upload image to my site. The image uploaded is shown as zero bytes with a broken image in the preview but uploading a non-image is fine.

Your environment is missing the package ImageMagick.

Mac OS:

brew install imagemagick

Ubuntu:

sudo apt-get install imagemagick