How to compress an image without resizing it?

[eluser]Aken[/eluser]
Compressing the file size / quality isn't a standard function of the image manipulation class. If I wanted to achieve this, I'd do the following:

1) Try to see if using the Crop function would work, by cropping the original photo to its original dimensions, using the 90% quality.

2) If that didn't work, I'd extend the image manipulation class and add a function designed specifically for reducing file size.

I can't test the first step right now, otherwise I'd do that before even writing this post. But I hope it helps somehow.