Derivative Image resize does not appear to work

Hello Folks
I recently upgraded to 2.1.4 from 1.5.3 and I am trying to implement the Berlin theme, but folks here are not happy with the size of the thumbnail images when you click on an item. I thought it would be quite easy to use the Derivative Image plug in to resize them but no matter what I set the Fullsize, Thumbnail, and Square thumbnail pixel size to in the Settings tab for the theme Appearance the images don't seem to resize. I found the errors below in error.log

2014-04-09T15:34:28-04:00 ERR (3): exception 'Omeka_Storage_Exception' with message 'Unable to delete file.' in /var/www/omeka/application/libraries/Omeka/Storage/Adapter/Filesystem.php:151
Stack trace:
#0 [internal function]: Omeka_Storage_Adapter_Filesystem->delete('thumbnails/f46a...')
#1 /var/www/omeka/application/libraries/Omeka/Storage.php(67): call_user_func_array(Array, Array)
#2 /var/www/omeka/plugins/DerivativeImages/models/DerivativeImagesJob.php(113): Omeka_Storage->__call('delete', Array)
#3 /var/www/omeka/plugins/DerivativeImages/models/DerivativeImagesJob.php(113): Omeka_Storage->delete('thumbnails/f46a...')
#4 /var/www/omeka/application/libraries/Omeka/Job/Process/Wrapper.php(29): DerivativeImagesJob->perform()
#5 /var/www/omeka/application/scripts/background.php(61): Omeka_Job_Process_Wrapper->run(Array)
#6 {main}

Any help is greatly appreciated.

Are you sure you have the permissions for the files directory and its subdirectories set correctly? That message is the plugin complaining that it was unable to delete the old thumbnail, which would usually only happen if there was a permissions problem. Are you able to successfully upload new images?

Note that some themes constrain the size of thumbnails with CSS, so its not always necessary (or effective) to change the generated pixel size of the thumbnails. You could also edit the theme to use the "fullsize" image if you wanted a significantly larger image.

Thanks John! That was it.. I was thinking that the problem was with the permissions in /var/www/omeka/application/libraries/Omeka/Storage/Adapter and not where you specified which was the right place to look

Thanks Much!