There is a commercial plug in package for Photoshop on the PC called PhotoFrame which puts fancy borders round images. The resource frame files supplied with this program appear to be large greyscale JPEG images and they can therefore be put to good use as Compo masks. Select the image you want to mask and open the horizontal mask editing pane. Holding down Shift drag the /FRM file to the pane. If the file has been settyped to JPEG a dialogue box will open immediately otherwise you will be told the filetype is not understood and asked if you want to load it anyway (choose Proceed and keep holding Shift until the scale dialogue appears).

scale mask dialogue
Choose Scale to sprite (Exact fit) and ensure that No mixing is selected as depicted in the image above and load the mask...

A future version of Compo may have optimised support for these frames with their own 'frame mask', along with faster loading and scaling.
Other things to try include loading a frame into the tint or shadow masks instead and giving the image a texture mask.
Many more decorative frames are available (at a price) from eFrames. The breathless sales pitch put me off a little though...

Better antialiased images from masked Draw and Artworks files

The following workaround produces good results if you've the memory. Load the drawfile into the bitmap layer, ensuring that the automask option is turned on and scale it to the desired size and angle of rotation.

Scale the image to either two or four times its eventual size using the scale dialogue.

Select the image and choose Edit 'drawfile' -> Properties -> Convert sprite from the main menu to remove the associated vector data. Scale the image back down by the appropriate amount (if you scaled to 4x size previously you'd now scale by 0.25). This is what you get....

Tricks with saving

Unlike many RISC OS programs Compo can usefully save to different bits of itself. You can use this feature to manipulate masks and images in all sorts of ways. A couple of be continued...

Web browser 'gotchas'

Progressive JPEGs appear darker in Browse than they do in Fresco. They also appear darker than non-progressive JPEGs (in Browse). Something to watch out for if mixing different sorts of JPEGs on the same page.