Completely forgot to give my feedback on this.
Your trick did indeed work, seen the guide before, but did not realise it was what I was looking for.
Regarding the syntax, especially with the conditions, how on earth did you figure that out?
<add input="##{C:1}##_{QUERY_STRING}" pattern="##([^#]+)##_.*id=([0-9_]+)" />
Is this common RegEx stuff? I would never have ended up with the correct solution here.
I can add that it is near impossible to validate via the GUI, so don't rely on that for everything. Trial and error seems to be the way.
For further reference, if anyone would be in the same needs.
<rule name="SillyRule" stopProcessing="true">
<match url="getpic\.asp$" />
<conditions logicalGrouping="MatchAll">
<add input="{QUERY_STRING}" pattern="picsize=([0-9]+)" />
<add input="##{C:1}##_{QUERY_STRING}" pattern="##([^#]+)##_.*id=([0-9_]+)" />
</conditions>
<action type="Rewrite" url="OrgBilleder/{C:2}.jpg?width={C:1}" appendQueryString="false" />
</rule>