milica: the bubbles are a bokeh effect and if you have g'mic installed you can use the bokeh filter there for a similar effect. if you don't have g'mic you might want to download a bokeh filter or plug-in for gimp - there may be more filters but i have used this:
free-form bokeh. i have also followed this tutorial:
heart shaped bokeh where you use the focus blur plug-in (has to be downloaded if you haven't got it already, i can't find the link right now) to create your own shape on the bokeh "bubbles".