Unfortunately it's part of a larger program already and tied to global settings. However, this is what I've done.

First I load the image:

pImageObject = BitMapObject, 
   BITMAP_SourceFile, DEF_IMAGE_FILE, 
   BITMAP_Screen, pScreen, 
   IA_Scalable, TRUE, 
BitMapEnd;

Then I calculate minimum size for the image (ulImageMinWidth and ulImageMinHeight) and use the minimum width and height as a "normal" image size, i.e. I make the image smaller. I noticed that this is required in order to scaling to work.

/* Set image size */
IIntuition->SetAttrs ( 
   pImageObject, 
   IA_Width, ulImageMinWidth, 
   IA_Height, ulImageMinHeight, 
TAG_DONE );

Then I create and open a window.

pWindowObject = 
   WA_PubScreen, pScreen, 
   WA_MinWidth, ulImageMinWidth, 
   WA_MinHeight, ulImageMinHeight, 
   WA_Width, ulWindowInitWidth, /* Make the window wider than the minimum width. */
   WA_Height, ulWindowInitHeight, /* Make the window higher than the minimum height. */
   WA_SizeGadget, TRUE, 
   WA_DragBar, TRUE, 
   WA_DepthGadget, TRUE, 
   WA_CloseGadget, TRUE, 
   WA_Activate, TRUE, 
   WA_GimmeZeroZero, TRUE, 
   WINDOW_Position, WPOS_CENTERSCREEN, 
   /* Gadgets */
   WINDOW_ParentGroup, VLayoutObject, 
      LAYOUT_AddChild, ButtonObject, 
         GA_ID, GID_IMAGE, 
         GA_RelVerify, TRUE, 
         GA_TabCycle, FALSE, 
         BUTTON_BevelStyle, BVS_NONE, 
         BUTTON_RenderImage, pImageObject, 
      ButtonEnd, 
   End, 
End;
 
pWindow = RA_OpenWindow ( pWindowObject );

Now when you resize the window, the image stretches accordingly filling the whole window.

Thanks but I don't want to prevent that. I have an image which is larger than the screen. The goal is to display the whole image in a window and retain its aspect ratio. User can resize the window and the image should resize as well: fill the window but keep the aspect ratio.

If for example the user resizes the window so that it's too tall for the image, image is scaled so that it fills the window horizontally and empty space is added above and below the image so that image's aspect ratio doesn't change.

The above code fragments make the image fill the whole window both horizontally and vertically, which is not good because the image loses its aspect ratio.

Thanks thomas!

I solved the problem by enclosing the image button with a layout gadget and modifying the layout gadget's LAYOUT_LeftSpacing, LAYOUT_RightSpacing, LAYOUT_TopSpacing, and LAYOUT_BottomSpacing settings when the window size changes. However, it doesn't work perfectly.

When you resize the window very fast, Reaction sometimes gets confused and fails to update the gadgets properly resulting oversized left or top spacing or distorted image.

Your image class example seems to work much better. If I can't fix the update problem, I'll use your code. Thanks again!