Hi Lis,
I think, it's not possible to change the size of this lines. But I want to show you a way to get equal spaces
First bring your pictures close together with the magnetic helplines.
Then use Alt+P to open the following dialog (don't know what it is in your language) to resize your pictures. If you want to have a space of 3 mm, decrease the size 0,3 cm as shown. Do it for both high and wide or first activate the option like shown. The pictures will be resized symmetrically and you will have an identical space.
.
Then bring the lines in right position also with the magnetic helplines.
.
I would not use shadow for the pictures. It is not realistic with a faded picture.
To decide, which font-size is the best, write some text in word with your preferred font in different sizes and print it.