I absolutely love my corner punches. I have the standard kind that simply rounds your corners. If I have cut pictures on the page into circles or ovals, I like to round the corners of my square or rectangular photos.
I also have two of the corner slotted punches. I use these on the mats for my pictures. One cuts two straight slots diagonally across each corner. The other cuts two scalloped slots diagonally across the corner. This lets you slip the corners of your photos into the corners of your mats. You can adhere the mat to the page, yet the photo is still easily removable from the page if you think you might ever need to remove it. (I really love this option for heritage photos---I hate permanently adhering them to a page.)
I have the Marvy Uchida brand of punches which can be found in any scrapbook store, Michael's, or Hobby Lobby. Hobby Lobby has their punches on sale for 25-50% off fairly regularly (at least here in my town) and Michael's is always running a 40% off coupon, so you can get these fairly inexpensively. The regular corner punch is about $8; the slotted ones are around $12 at full retail price.