Scribus has some nice features in editing photos. Generally, you will prefer do that in a photo editing software where you'll have more choice in the functionality and algorithms. However, being able to do some fast, last minute changes is really handy. Plus it will help you to get better results even if you don't want to learn another software.
Image effects can be applied on Image Frames from:
The Image Frame contextual menu Image Effects
The Image tab of the PP; click on the Image Effects button
Extras | Manage Images has an Image Tools tab that includes an Image Effects button too
There is a little list of effects and they are very basic operations. You apply an effect by selecting it in the left-hand column and then clicking on the >> button. Applied effects are listed in the right-hand column. You can apply as many effects as you need, but generally two or three will be enough. If the effect you chose has some parameters, they appear under the image preview, on the...