Tips for locating (and deleting) iPhoto images
Let's start with EXIF data. Images you take with a digital camera have metadata implanted in them. Using iPhoto's Search field you can, well, search through this metadata. By doing so, you can identify (and possibly eliminate) certain images.
For example, I want to find all the images shot with the front camera on an iOS device. All I have to do is enter the word "front" in the search field and there are some not-terribly-flattering images of a cat sitting on my head. If I click Info you can see some of the EXIF data at the top--the word "front" appears here and so is searchable.
Or let's look for photos taken around the holiday season in 2012. To do that I click on the Search field triangle and choose Date. I then navigate to the year 2012. Any months within that year that contain images appear in bold text. If I click a month, I'll see any images taken during that month.
But I can narrow my search by clicking this small triangle to see a month view. Any days that have images associated with them are likewise in bold. I can see a range of dates by dragging on dates in the calendar or choose non-contiguous dates by Command-clicking on them.
Or let's say you want to find any videos shot with an iPhone or other camera that uses the H.264 video format. Just enter H.264 in the Search field and they'll appear. And while have them, press Command-A to select all of them and then choose View > Keywords. In the Keywords field that appears, enter the word Video and press Return. Now, when you want to search for your videos, you can instead just enter the Video keyword in the Search field.
And then there are smart albums. The Search field is great, but it allows you to search for one thing only. If you'd like to search by multiple factors, choose File > New Smart Album and start creating conditions.
For example, if I want to find iPhone 5S images shot at a high ISO (meaning that I was shooting in a dark environment and so the images are likely to be grainy or underexposed) I'd choose Camera Model is iPhone 5S and ISO is greater than 800.
If I then click on the Photos menu and hold down the Option key I can choose to move any selected images to the trash.
I can use a date trick here as well. I could search for images taken during the recent holiday season with my Nikon DSLR by configuring a smart album that has conditions that read Camera is Nikon D750 and Date range is 12/15/2014 -- 12/31/2014.
The thing is that digital cameras and devices like the iPhone make it really easy to take pictures, and that's great. But with that power comes the responsibility of finding new ways to locate the images that are most important to you.