Hi,
Based on your error log, it sounds like DSpace cannot find ImageMagick. Either it's not installed or you haven't set the "org.dspace.app.mediafilter.ImageMagickThumbnailFilter.ProcessStarter" setting properly.
The important error is this one:
Caused by: java.io.FileNotFoundException: convert
That is telling me that the ImageMagick "convert" script is not found.
Tim