An image in memory takes often much more space than on a disk. The reason: images on a disk are compressed. To create new sizes or apply other changes to the image it has to be uncompressed and depending on color depth and compression algorithm.
There is no way to predict the real memory usage before the image is uploaded.
What you can do before upload:
- Reduce the color depth.
- Cut the image down to the maximal size you really need. Do not upload an image with a width of 2000 pixels if your maximal width is 900 pixel.