You can already do that with a cutout/frame.  There are some included with Qimage but you can create your own that have any style including drop shadows, wooden frames, etc.

https://www.ddisoftware.com/qimage-u/help/function.htm#cutouts

Regards,
Mike

I've created a cutout frame template in tiff format (length/height = 3/2) and it works. But this solution has probably a drawback with the stretching of the frame. If the image isn't exactly proportional to the cutout template in the length/height ratio, the frame will be stretched dis-proportionally, i.e. the distances of the outside lines from the image are not the same on the top and bottom as on the left and right hand side. For example: the cutout with a 3/2 ratio applied on an image with a 2/1 ratio results in a distance of 2mm on the left/right, as defined in the cutout template, but is reduced to less than 1.5mm on the top and bottom. Depending on the amount of stretching this can be quite visible.

I could create a series of cutouts which fit the most common image ratios to avoid very visible distortions but since I print for many other people, images come often with freehand cropping. So I cannot accommodate all ratios. Wouldn't a photo mat solution not have this problem? It stretches the mat out of the image according to the settings in the dialog box, right? 

I'm not sure it's really a creative frame, it just simulates a 2mm thick card board passepartout. Usually people give me an image and specify the length of one side of the printed image without the frame. Then, to avoid the hassle of precisely cutting a card board around, I was asked whether I could simulate the passepartout by printing this frame outside the image.

Anyway something seems not to work anymore, I haven't figured out what. I created a frame in Affinity Publisher and exported as tif. It worked fine. Then, based on this one, I adapted it to various frame  sizes (square, 1/2) but those don't work anymore. They show up in the cutout list but they are not applied and when clicking Done and saving, the thumbnail disappears in the main window.

That's correct.  Remember that cutouts and frames are image edits and not print add-ons so they are applied to the image.  You could then decide to print that image at any print size so the frame that got "stamped" on the image gets resized since it is part of the image.

Attached is the change: new on the top, old on the bottom.  I intentionally created a very wide image and then applied the wood frame.

P.S.  The next version will have improved frame logic so that even though the frame might get scaled like the image, the edges don't get distorted if your image differs significantly from the frame design WRT aspect ratio.  So that will make one variable easier.

Regards,
Mike