Link to an image on a web page

You're in luck, that already works:

[""http://www.vornberger.net/medien/jan"".jpg]

You can even resize remote images:

[""http://www.vornberger.net/medien/jan"".jpg?200]

Thanks Jendrik Seipp, that solved my question.

That could now solve my weather problem, provided, I find a webservice, that - given a date - generates a weather symbol.

Indeed, that would do the trick. You can specify the URL for the
webservice in a template and use the $date$ macro to insert the current
date when you insert the template. You can set the date format that the
webservice needs in the preferences. An example template could be
http://fictional-weather-api.com/$date$/california/image

That would have been my next question: What is the $date§ macro ? What I need is a String, that I could concatenate to the (still to find) servers url and that evaluates to the current date.

Try putting "Now: $date$." in a template and insert it. $date$ will
expand into the current date. The format for expansion can be set in the
preferences.

OK. That works, HOWEVER: it is the current date ! Since I have to insert this macro with the link to the web service by hand into a lot of days, it would be important to have a macro that expands to the day you are editing.

Indeed, that it where the macro system would come in handy...

Sorry. Did you say "the macro system would come in handy". It expands to the current data, and not the date of the day the macro is inserted, right ? And that doesn't help, right ?

Thanks Jendrik Seipp, that solved my question.