Nermander, you're correct but only if the OPs requirements are met, which they can't be because Scribus doesn't work that way (see below).
a.l.e, the OP doesn't want to have a margin - padding - inside the image frame. They specifically say they want "the images to exactly fit the frame".
The issue, as I see it, is that there is the requirement to add image frames which all have the same behaviour where there is a consistent gap around the frame which defines how far away text flows around the frame no matter what shape and size the frame is.
Using the Frame Shape does not allow this, and neither does using the Bounding Box. The Contour Line allows this for a specific instance but the contour line, once edited, is automatically changed by Scribus proportionally to the size/shape of the bounding box of the frame.
This means that when the user edits the contour line for a frame, then duplicates that frame, then changes the size/proportions of the frame, the contour line will change proportionally too. They then have to edit the contour line for each frame. This is not what the user wants.
This is why I suggested we have a "margin" setting where the user can define a gap outside the frame that will stay a consistent distance no matter what the size and/or proportions of the frame.
This could be as simple as having an extra "Use Margin" text flow option - in the Shape tab of the PP - with a setting in Preferences where the user can define a preferred margin for all frames.
It could get more complicated than that but so far there have been no stated requirements for that extra complication. All the OP wanted was "to get 2mm spacing on each side of the image" as they "usually want to have the same properties for each image frame", so an application-wide setting would suffice for this and most uses. Having an individual setting for each frame would give the user more control but it's not been asked for yet.
Personally I don't think this is a particularly strange thing to ask for and implementation shouldn't be much more complicated than taking the existing bounding box, adding a specific distance to each side, and using that to control the text flow. Scribus already contains most of the code to do it - the contour line does this but in a slightly different way - it just needs a little bit of tweaking to do it a bit differently.
pck1980: If I have misrepresented your views/requirements in any way then I apologise. Maybe, as a.l.e suggested, you could attach something so we can get a better look at exactly what you want rather than just inferring it.