Use this one Julien: https://appstore.home.mendix.com/link/app/5958/TimeSeries/Set-Attribute and set the Alt attibute on your image.
Hi, any news about that?
Keep an eye on our blog: http://appronto.nl/over-appronto/blog/userfriendly-screens-mendix/
We will release a widget soon which can achieve this. It can manipulate any attribute/node in the HTML.