✔ 最佳答案
Using Photoshop Elements, just crop to any square size. 5x5 is one of their presets. Next, resize the image. Hit [CTRL]+[ALT]+[I] (That's a letter "i") and type "300" in the space for pixel dimensions. If the proportions are constrained, it will change both and you'll be all done.
Alternatively, you can choose the crop tool and for the dimensions, set 300 px in one box and 300 px in the other. Whatever area you demarcate with the crop tool will result in a 300x300 pixel photo.