Adding transitions changes picture/video footage times

This is expected behavior, however frustrating it might be. You might want to become familiar with preference settings. Some apply just to your current "project", where you can set things like how long photos play, the length and type of transition and so on.

I find it useful to have the automatic preferences set while I put a project together, so that whatever transition and slide timing I prefer is applied to everything, and then turn off the automatic feature so I can change the individual details of just those pics or transitions that need it. This way, I don't have to change nearly as many once everything is in line.

While editing a project, choose Project Properties from the File menu (Control-J). The automatic addition of transitions is controlled by the checkbox on the "General" tab. Control of the transition timing is on the "Timing" tab.