How many photos do you have in the trash? What happens when you try? How much free disk space do you have? Why can't you move any photos to the external drive?
Remember we know nothing about yoru system except what you tell us - details are critical to getting detailes advice.
It is likely that the solution will include running iPhoto with th elibrary on an external drive
Moving the iPhoto library is safe and simple - quit iPhoto and drag the iPhoto library intact as a single entity to the external drive - depress the option key and launch iPhoto using the "select library" option to point to the new location on the external drive - fully test it and then trash the old library on the internal drive (test one more time prior to emptying the trash)
And be sure that the External drive is formatted Mac OS extended (journaled) (iPhoto does not work with drives with other formats) and that it is always available prior to launching iPhoto
And backup soon and often - having your iPhoto library on an external drive is not a backup and if you are using Time Machine you need to check and be sure that TM is backing up your external drive
I temporarily solved the problem by restoring the 150 photos to the library and sending to trash a few at a time. I have 13,000 photos in iPhoto and am trying to off load them to an external hard drive. I have never used Time Machine so will read about that. Thanks. I'll copy this and try it when I have a few more pix deleted!
To move iPhoto to an external drive:
Make sure the drive is formatted Mac OS Extended (Journaled)
1. Quit iPhoto
2. Copy the iPhoto Library from your Pictures Folder to the External Disk.
3. Hold down the option (or alt) key while launching iPhoto. From the resulting menu select 'Choose Library' and navigate to the new location. From that point on this will be the default location of your library.
4. Test the library and when you're sure all is well, trash the one on your internal HD to free up space.
It may be better to move the library first - deleting a few (even a few hundred) photos does not recover much space
One possible quick space saving is to right click on the iPhoto library and show package contents - if there is a iPhone Cache there you can safely delete it - make NO other changes- and it will be recreated on the next sync - it can get large and might help short term)