Startup disk is full, but i've deleted everything, and it's still full..?

I have a macbook from mid 2010, and it keeps telling me my startup disk is full and i need to delete files to free up space. I've since then deleted thousands of photos i had on my mac, and moved them onto my external harddrive. i've deleted them from iphoto, and iphoto trash, and also emptied my mac trash. I've emptied my cache, i've deleted all safari saved things, and i've cleaned my mailbox too. Nothing seems to do anything. I now can't put photos on my mac at all as it says i dont have space for these.. im a little confused and upset that nothing is freeing up space even though i have deleted gb's worth of files.

Any help on how to free up this space would be greatful.