Hi,

I’m doing also manuals (utility vehicles), graphics are mainly not  
screenshots but drawings (or photos), but I have also chapters which  
consist mainly in screenshots (of a 640px or so  control device).
My messures refere to A5 landscape.
Best solution for me so far: a constant margin of about 45 mm where I  
can put my images/screenshots in; the images go in as  
\place<float>[margin]; so they are always aligned on top and  
respecting each other;). Sometimes I do a  \margintext or \inmargin  
but preferably vers the end of the production.
It happens that on some pages the margin will be empty. For me this is  
ok.
The advantage is, that you’ll rest quite flexible, when you have to  
insert new things or to replace images. The flow-text flow is quite  
good.

In annexe I put my env-file (a bit cleaned-up), so you can get an idea  
(once again: its a5 landscape).