While kdenlive might not be the product of choice in this area, I was wondering if the following could (easily) be accomplished:

For example, I've been on vacation, took a bunch of pictures and, rather than making a boring slideshow out of them, I want to select the best pictures and make a movie from them.
Now, to make things a bit less boring, it's fun to have the pictures show up in a few different ways: slide in from the top, bottom, left or right, fade, dissolve into the next, turning like a fan, etc....

When looking at the effects list, there seems to be no effects for this readily available... I might be wrong of course...

Would it be hard to create custom effects for these things? And how would one go about this? E.g.: there might, or might not be a tuturial available on this subject. Please point me to where I should look for answers. (or tell them straight out ;-) )

Take a look at the transitions. This is what you are looking for.
To add one right click on a clip in the timeline and add a transition.
Now place the next photo beneath the transition or select a track to apply the transition on.

That is brilliant; Managed the dissolve into each other perfectly now... Will play with other transitions soon. Thanks for that tip; should've thought of having different tracks is good for stuff like that ;-)