If you are using iPhotos, In the sidebar of iPhotos you’ll see a Trash can. Right click it and select delete. Until you actually empty the trashcan nothing is deleted from iPhotos. If you are using Photos the pictures aren’t immediately deleted in case you want to change your mind. It takes about a month. If you select the Recently Deleted folder you’ll see under each picture how many days are left before that photo will be permanently deleted.
I mention both because you specified iPhotos in your question but I don’t recall it having a recently deleted folder. Photos does have a recently deleted folder. If you are referring to Photos you can speed up the deletion process by logging onto your iCloud account and selecting the Photos app there. Then select Albums. Open the Recently deleted folder. At the top of the browser window to the right you’ll see Delete All. Click on that and it will delete all those photos permanently. This should restore the space on your computer. When my Photos library was totally screwed up this is how I started all over.