How can i change the behaviour of the uploader.

When i upload an image it is changed from 72 DPI to 96 DPI increasing the file size.

It reduces to 750 pixels as set in the options of the photo section. But why does it increase the DPI value?

I am not sure I am following you here.  DPI, dots per inch, is a printer resolution, not a monitor resolution.  DPI is the number of dots that can be placed within one inch of paper space.

Yep.. a DPI setting within a digital photo is meaningless, and has nothing to do with the file size of the image.  If the file size changes after uploading, it's because of resizing or a different compression ratio.

It's fixed. The GD library that is used by dolphin cannot change the DPI. It has a fixed value. It cannot be changed in dolphin. It's part of the GD library that is in turn part of set of PHP packages installed on the server.

It does not matter anymore anyway. The most modern graphics software does not use DPI anymore. DPI is use for printing and even 96 is extremely low and would result in tiny images if you were to actually print it on paper.

DPI is nothing more than a conversion factor that determines how to resize a screen pixel to represent a dot on a printer.