Photos and videos imported to iPhoto cannot be found

Sounds like you made a bad decision to use a strongly not recommended referenced library

With iPhoto you can reference your original photos rather than copy them to the iPhoto library - but it has many problems and is strongly NOT recommended

Simply go to the iPhoto preferences and uncheck the default preference to "copy imported items to the iPhoto library" - now iPhoto will link to your external photos rather than copying them

HOWEVER

iPhoto is very poor at fixing these links if the path changes so replacing failed hardware and upgrading hardware can be very difficult

Importing becomes a two step process - once into your file structure and a second time to iphoto

Deleting becomes a two step process - once in iphoto and then in your file structure

And if you make any modifications in iPhoto ( including having the auto rotate flag set by the camera) a modified version will be created in the iphoto library and this will not be reflected in your file structure

see also iPhoto and File Management

If you have a referenced library then you must always leave the source photos exactly where they were when you import them to iPhoto - for your problem you need to put them back where they were - hopefully you have a backup of them

LN

Thinking they were safely stored in the iPhoto database, I then deleted the folder on the desktop containing the uploaded items.

Sarah, did you have time Machine running while the image files were still on your Desktop? If you are lucky, they may have been backed up by time Machine, before you deleted the photos. And just to be sure - you checked, if the photos are still in the Trash in your Dock?

If you unfortunately really emptied the Trash as well, before Time Machine could back up the photos, your only hope would be expensive data recovery software. Most of them have a trial mode, so you can see, if any photos can be recovered, before you purchase the software, for example Disk Drill: http://www.cleverfiles.com/

By default Photo uses a managed library - to have a referenced library you have to intentionally uncheck the option

If you have a referenced library then you must always leave the source photos exactly where they were when you import them to iPhoto - for your problem you need to put them back where they were - hopefully you have a backup of them

Note that everything you imported while using a referenced library will have this problem so do not delete the source photos for any of tem either

LN