Hello Michael,
in addition to Uwe's hint, about the screenshots, I want to put in my five cents too. If I understand you in the right way, it's a question of figure out the geometric proportions, what means you have to give blind colored areas (edges) around some of your images.
To adapt images in one go into your wished measures, you will find some efficient instruments by using Google. My favorite in this case is "PicSizer".
Only as an example for arranging thumbs around an image, please have a look onto one of my websites, which I made in DW:
Rundtour um das Kaiserjoch im Lechtal. By moving the mouse the grater image appears in the middle.
Hans-Günter
P.S.
Könnten wir ggfs. in Deutsch miteinander kommunizieren?