Can't add multiple / large items

I'm having issues every time I try to add a large file or multiple files to an item. It doesn't matter if I use the Dropbox plugin or the CSVimporter plugin. I'm able to add one or two small (e.g. 2 MG) files without problem. In the past, I was able to add files ~15 at a time, but any more than that would usually cause problems. The large files aren't unusually large, but if I'm trying to upload very many it always fails.

I've edited the "Fullsize Image Size" option in the general settings to a ridiculously high number in the hopes that was the problem. Is there anything else I can do? Imagemagick seems to be configured just fine.

The error message I receive is "Omeka_File_Derivative_Exception Something went wrong with image creation. Please notify an administrator." (The most recent example was an attempt to attach 6 jpegs for a total of about 45 MB). Here's the error log:
#0 /home/pplspc/ Omeka_File_Derivative_Image::createImage('/tmp/6978ffe390...', '200', 'thumbnail')
#1 /home/pplspc/ Omeka_File_Derivative_Image::createDerivativeImages('/tmp/6978ffe390...')
#2 /home/pplspc/ Omeka_File_Derivative_Image::createAll('/tmp/6978ffe390...', 'image/jpeg')
#3 /home/pplspc/ File->createDerivatives()
#4 /home/pplspc/ File_ProcessUploadJob->perform()
#5 /home/pplspc/ Omeka_Job_Dispatcher_Adapter_Synchronous->send('{"className":"F...', Array)
#6 /home/pplspc/ Omeka_Job_Dispatcher_Default->send('File_ProcessUpl...', Array)
#7 /home/pplspc/ File->afterInsert()
#8 /home/pplspc/ Omeka_Record->runCallbacks(Array, Array)
#9 /home/pplspc/ Omeka_Record->save('afterInsert')
#10 /home/pplspc/ Omeka_Record->forceSave()
#11 /home/pplspc/ Item->saveFiles()
#12 /home/pplspc/ ItemBuilder->addFiles()
#13 /home/pplspc/ insert_files_for_item()
#14 /home/pplspc/ dropbox_save_files('Filesystem', Array, Array)
#15 /home/pplspc/ Omeka_Plugin_Broker->callHook(Object(Item), 'Filesystem', Array)
#16 [internal function]: Omeka_Plugin_Broker->__call(Object(Item), Object(ArrayObject))
#17 /home/pplspc/ Omeka_Plugin_Broker->before_save_form_item('dropbox_save_fi...', Array)
#18 /home/pplspc/ Omeka_Record->runCallbacks('before_save_for...', Array)
#19 /home/pplspc/ Omeka_Record->saveForm('before_save_for...', Array)
#20 /home/pplspc/ Omeka_Controller_Action->editAction(Object(Item), Object(ArrayObject))
#21 /home/pplspc/ ItemsController->editAction(Array, Array)
#22 /home/pplspc/ Zend_Controller_Action->dispatch('beforeSaveForm', Object(ArrayObject))
#23 /home/pplspc/ Zend_Controller_Dispatcher_Standard->dispatch(Array)
#24 /home/pplspc/ Zend_Controller_Front->dispatch()
#25 /home/pplspc/ Zend_Application_Bootstrap_Bootstrap->run()
#26 /home/pplspc/ Zend_Application->run('editAction')
#27 /home/pplspc/ Omeka_Core->run(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#28 {main}

Do you control your server, or are you using shared hosting?

This could be a problem where you aren't allowed to use over a certain amount of storage space in your server's temporary directory, which you're going over when you upload big files or many files.

Sorry, I should have mentioned that I'm using Dreamhost shared hosting.
Can I set a different temporary directory through Omeka, or will I have to do it through a php.ini file?
Thanks again for the help.

You should be able to change the temporary directory Omeka will use for making image derivatives in Omeka's application/config/config.ini file.

The setting you want is called storage.tempDir, so you'd want to add a line like

storage.tempDir = /path/to/your/new/dir

You'll need to create the directory you want to point that to, and give read/write permissions for the web server (it should have the same permissions as the "archive" directory).

Thanks, John. I didn't have much luck with getting the storage.tempDir setting to work, but I did have success with changing the temporary directory in the site's php.ini file. Thanks again for the help.