Hi @CarlosSousaFerreira following discussion with colleagues, it turns out it is possible to concatenate strings in an expression in the report template itself, using the plus symbol (+). So your report template syntax could look something like:
${$image | src:"https://anyserver.com/dynamicpage.ashx?data=" + my_field}
Note that this method is not documented; we're still assessing the best way to implement this syntax, but this should hopefully be useful for now.
Also note that when calling a question value within an expression, you drop the ${} syntax from the question name. So in this case ${my_field} is entered as my_field when inside the ${$image} expression.
Best, Jim